Hey there bigbutterbean,
I went a similar route with my current project, though on an old school schwinn cruiser frame. I came across a few obstacles, but, think you won't encounter as many. For mine, first off the older schwinn's have an oddball diameter seat post. I couldn't find one locally, I so made one of my own. I wanted to upgrade to the modern style bottom bracket with sealed bearings, so I ditched the one piece cranks and welded in the bottom bracket off of an old mountain bike. You can get easily get the adapter to convert, but I like to weld and had an old bike at hand for parts. The sealed cartridges get my vote. Another issue you may come across... if you have a coaster brake, you will have to convert to a handbrake. For me, minimally, I wanted V-brakes. I had to weld on brackets in order to do so. Also, lastly not least, unless your rear wheel shifts gears internally, you will have to think about how you will mount the rear derailleur. My old frame had no provisions to mount one. There are bolt on versions out there that attach to the axle itself, but I opted to weld on a tab and tapped it with threads. Going this route will allow me to upgrade at will should I choose to go with more gears in the future. Hope this helps.....
~ evil ~