My thoughts:
Cranks: 1 piece for economy and tradition. If you go 3 piece, go bmx. If you like to make your life complicated in pursuit of weaker parts, go euro-adaptor with a square-taper or newer crank/BB interface.... added bonus: it'll cost more, too! You've made a conscious decision to build a frame with an American BB shell for trail-riding; why not go with a crank designed for use with an American BB?
Hubs: Coasters are fun. Morrows were top-ranked in the 70s offroad scene; contemporary klunker enthusiasts with a sentimental flair love the Bendix Red Band, which is a great (albeit sloppy) hub, but the practical move is to run a Shimano cb-e110. These hubs have their problems, too (clutch spring in particular is weak, and they don't modulate well... lots of drag...) but they're cheap, plentiful, current, and therefore, they are easy to find parts for. Fact: klunking kills coaster brakes, ad but true. A lot of quality high-temp grease and proper adjustment helps, but you WILL kill your coaster, especially the guts.... nothing is easier than swapping everything from a fresh hub into your wheel's hubshell. That gets expensive with Bendix, plus time-consuming, finding good deals on Bendix hubs with life left in'm.
All that being said, my fave vintage coaster is the Fichtel&Sachs Komet Super.
Threaded vs Threadless: Choose your fork first, then go from there. Be forewarned; Worksman uses its own weird-spec headset cups. PErsonally, I like to run a chromo 1.125" threadless mtb fork, like a Surly Instigator or 1x1 fork. To run that, you can keep the oem Worksman cups and cannibalize a loose-ball NECO or SUNLITE 1.125" threadless set, or get yourself a nice headset and send it off to
@chattymatty or Mr.Tick to be turned down to fit the Worksman headtube. (I got a set from ChattyMatty; I have the exact specs on here somewhere, let me know if you go that route and i'll dig'm up...)
If you go 1", go threaded. Save the oem cups but buy the VP 32.5mm headset from Porkchopbmx.com
http://porkchopbmx.com/vp-h755-bmx-bicycle-headset-1-threaded-w-32-5mm-cups-chrome/ and use allllll the other parts from that, except the cups. Also, order up both sizes of crown race, b/c it seems like half these forks take 26.4 and the others take 27.0..... running 1" threadless isn't exactly impossible, but it presents a whole lot of drawbacks and absolutely no advantages, so i can't recommend it.
The Worksman is a fine choice for klunking; the geometry is nice b/c it's really slack, with a nice high BB height. You'll love it. The main weakness is the puny rear dropouts, but the frames are so cheap and attainable, i don't even really care.
Final thought:
@us56456712 i agree that straight 6s are rad; i think the best is the Ford 300, but the main reason the world moved to v6 instead was front-drive. Running an I6 of decent displacement transversely presents packaging problems. And, with rear drivers, parts-sharing makes it a lot easier for manufacturers to just run the v6 longitudinally in the rwd car.... BMW doesn't make front drivers, so it's easy enough for them to keep the I6 going, but with the realities of the modern aut market, most everyone else has moved on, for the most part....