I'm planning to do this somewhere down the line (currently into later decades with my projects), and suspect that if I go the "resto-mod" route that I'll likely use a Schwinn Varsity frame and fork to start things off. The welded construction and geometry are about as close as one can get to an early 20th century frame without actually purchasing an antique. The bonuses include them being inexpensive and compatible with many modern components without quirks.
Good luck!
-Gregory
edit: There's a long thread at Rat Rod Bikes where many folks show off their "Pashley" style resto-mods, many of which are actually quite inspiring. Check it out.
http://ratrodbikes.com/forum/index.p...e-bikes.23043/