Great plan, but you're including one totally unnecessary step (installing the OPC), and you're also planning on a time- and labor-intensive step (building the wheel) that you could save for later. Sure, you're going to have to lace the wheel at some point, but think about your order of operations; it'd probably make more sense to lace the wheel once you've ordered the proper size BB. (Sometimes, shipping takes forever.
)
You can measure the hub's chainline by measuring the distance between the outer edge of both jam nuts (often referred to as OLD, or the O.ver L.ocknut D.imension)... I don't have one in front of me, but i think it's something like 112 or 114mm. Now, measure from the rear sprocket's teeth to the driveside jam nut; take that figure, and subtract it from half of whatever your OLD measurement is, and that's your chainline. I'm willing to bet that it's going to be 41 or 42mm.
This method will work, assuming that your frame is aligned, and there is no deliberate offset, which there shouldn't be on a Schwinn canti frame....
As for finding some clues on your crank and what spindle it needs, i'm hoping that your bb shell has no cups or adaptor in it. You said you have an OCC BB set, which is good b/c it has a JIS square taper spindle, and it's loose ball, so you can separate the spindle. If you have a crank-puller tool, you could install the driveside crankarm (with your intended chainring mounted to the spider) on the spindle, line it up by hand so you get your desired chainline, and via careful measurements, you could figure out how long the driveside half of the spindle would need to be to get that chainline. There's a lot of ways to do this; i can think of a few but I'm getting the sense that you wanna do this your way-- just remember that guesswork won't cut it.
While you're mocking this up to find chainline, please do make sure that the crankarm isn't going to hit the chainstay when the crank is on the proper chainline. It may not, but it very well might, if you're using a crank from a lightweight/road bike on a cruiser.
Once you figure out what the driveside half of your spindle needs to be in order to achieve chainline that matches the rear, double that figure (in mm, trust me) and then buy a
symmetrical BB set in English-thread 68mm x ___mm (whatever your final measurement yields.) If they don't have the exact size, get one within a mm or 2, or see if there's an
asymmetrical set available that'd suit your needs, and keep in mind that you can get a few more mm inboard or outboard by switching which side of the spider you use to mount the ring, or even using a few washers to space out the chainring bolts, like
@OddJob mentioned.
(Note-- many coaster hubs and IGHs take 3-tab sprockets; you can adjust rear chainline by using dished sprockets, and in many cases you can run the dish either inward or outward, or run a flat, so there's a few mm to play with there, too. The Bendix kickers, however, have one-piece drivers, and don't allow swapping of the rear sprocket, as it is actually part of the driver.)
Incidentally, buying the Sunlite conversion kit may or may not have avoided these problems; those kits come with a 127mm spindle. The spindle itself is threaded, so you can adjust the front chainline within a few mm based on how you set up the spindle, but that adaptor won't work with most modern cranks. You didn't say which cranks you're running---there were many Schwinn-branded cranks over the years, and it'd be helpful to know what model bike it came from-- but it sounds old school..... If that's the case, my best
guess would be that you're gonna need a 125 to 127mm spindle in the end. So, in this case, that may've been the more expedient part to run.
