1-If it's prewar and solid enough to actual klunk with, it'll likely be both expensive and kind of a shame to chop, imho. If it's a rotten hull, then it won't be a shame, but it wouldn't likely be trailworthy. At any rate, prewar schwinns seem to command a premium price, but anything with a double-top tube structure seems to be pretty dear.
2-common? inexpensive? As I said above, anything prewar, straight, and solid is not going to be easy to find, especially not for cheap.
3-Head angles are likely to be slack on most of the older Ballooner bikes that folks tend to gravitate towards for these builds. Changing the headtube on an old American-made bike is an exercise in pointlessness; the headtubes tend to be 32.5mm or greater ID. Depending on what frame you're using, you can buy a retro ryder headset (
http://www.genuinebicycleproducts.com/parts.html) (sscroll down a lil more than halfway.) Or, you can use a wald (or similar) headset's cups with a cheap, caged bearing 1.125" threadless headset to make a hybrid headset. In either case, you'll save time and money over replacing the headtube, and then you won't need to feel bad about modifying a vintage frame, making resale more likely... (And, yeah, I know that a lot of ppl like to take on chop/weld projects, so they can brag/gloat, but you won't impress anyone who knows anything, as you won't need to do all of that to run a 1.125" headset, and you'll likely weaken the frame...)
Geo. Yeah, the modern mtb has it's origins in the classic balloon tire bikes, but in reality, they differ quite a bit insofar as they have very slack head angles, very long chainstays, and most have a 17 to 18.5" seat-tube, so if you tend to run a "medium" 26" hardtail, you're good... otherwise, not so much. Tend to have more BB drop, too. And, shorter toptubes. Maybe therein lies your chop project (ie, try to chop the cruiser and give it the same geo as an On-One Inbred, for example), but if i wanted something with semi-modern mtb geometry (or even just NORBA geometry), I'd buy a steel hardtail from the 90's. (I have a ca2000 Kona like that.) If I wanted to build a klunker (and I have, which I'm revamping, with another klunker build in the works...), I'd embrace the offroad cruiser vibes, with the iffy geo and all. FWIW, I actually enjoy riding a Worksman INB offroad; the geo works well for xc-style trails, if you're not in a hurry.
Aside from bragging rights, I don't see the need for a very old frame. Especially if you're going to run a suspension fork on it. (The suspension forks & 69er wheel layout will totally negate the ol-school vibe of a vintage frame anyway, but perhaps you're after the anachronistic appeal, which I sort of understand....) Especially if you're looking for something solid-yet-worthless to tinker with, chop, weld, etc on before hitting the trails for actual trail riding. Therefore, my suggestions are:
1-
Worksman INB. These are dime-a-dozen, current production (but are very nearly identical to Worksmen from 60 years ago), overbuilt lugged frames,with the same twin top tubes as the DX/Excelsior Schwinns that are often considered the quintessential Klunker frame.
Pros- will take a 1.125" threadless fork with a retro ryder or hybridized headset, they have all of the real-deal oldhead USA bikes, including track ends (rearward-facing drop-outs), american BB shell, overbuilt lugged construction... most ppl will have no idea that it's a current-production bike. RRB member "Ind-chuckz" often sells framesets on this site for $70 or so, shipped. The geo and ride characteristics work very well on my fairly flat S.Jersey trails; ymmv.
Cons- The curved downtube doesn't scream "klunker" the way a straight one does. Flimsy 7/8" seatpost (Bendable, but still stronger than most true-vintage options.) The frame weighs more than my Mom, who is light for a mother, but quite heavy for a bicycle. Worksman seems to paint these with watercolor paint, so even a 2 year old frame will be loaded with surface rust (which may add to the faux-vintage factor); the tubes are very thick-walled, so they will tend to be solid under the surface.
2-
late-model Schwinn Heavy Duti (mid-1980's to present) As someone else said, these are a good choice.
Pros Geo is very MTBish; very similar to my 90's-lookin' Kona, but with longer stays and a slightly slacker headtube. Downtube is straight. Made from oversized tubing, so it's stiffer than the Worksman or any other frames you're likely to consider. Comes with a 1.125" headtube standard (although the stock fork is threaded). Comes with slightly wider (116mm) rear spacing, in case you wanna run gears. Has horizontal drop outs, american BB shell, and takes a 1" seatpost, which is basically the bmx standard, with a lot of options. Also, 1" is way stronger than the 7/8" on the Worksman or the 13/16" on Chicago Schwinns (even thinner on many other prewar bikes...) Ind-chuckz always seems to have these available for about $75 shipped.
Cons: Only a total yutz will think it's an actual vintage frame. Sure, it has the twin top tubes and some other old-school vibes, but it's constructed of tig welded OS tubing. These are also pretty heavy, but lighter than a worksman, an electro-forged schwinn, or most other frames suitable for klunking.
If you really have your heart set on something truly old, your best bet is to look for something straight, solid, and in your price-range, and deal with whatever issues you need to solve to make it how ya want it. Having your heart set on a particular vintage frame in decent shape will probably be costly and may be time-consuming. If you can live with a newer frame, you'll find these are more easy to come by, and usually dirt cheap. Whatever you decide, I'd urge you to explore the headset strategy more thoroughly before you chop. You might also want to build and ride the frame as-is so you can decide if you really need to mod the geo, and if so, what direction you want to take with that. Also, rethink the suspension fork thing; good suspension forks will kill your budget, and lousy forks will kill your build. Rigid forks are cheap, strong, and the type of thing that the real Marin guys used to run.
Sorry to write a book; as you can see, I get pretty enthusiastic about these kinds of builds. The above is based on my own research and experiences, and assumes that you'll want to build a bike for actual trail-riding. If you just want to make a cool-looking faux-klunk for pub rides, I totally respect that, but my advice will be pretty useless for that.
Above all else, build whatcha want, how you want it, and have fun doing it.
-rob
