RetroBlackRifle has oodles of good information but its forums haven't seemed to catch on like the Retro forum in ARFCOM. Plenty good information stickied there too and much more active.
http://www.ar15.com/forums/f_3/123_.html
S.B., there are some mostly complete A1 parts kits floating around various places. A new lower receiver and a new barrel and you're in business. Bounce over to ARFCOM for lots of inspiration. Lots of them there are all ate up with NODAK Spud lower receivers for authenticity, but if you don't mind an A2 lower receiver will work just as well and so will a fast twist lightweight barrel from somebody like DEZ arms. You can get that done for half to 3/4 the cost of a decent SP1, especially if you can do most or all of the work yourself.