Well, I found a way to convert my 64 to power disks, 5-lug wheels and get a 12-bolt posi, all at the same time: Buy a 74 C10:
http://www.chevyasylum.com/74c10/74c10.html With a 454 and TH400 no less...and a buncha new parts. Oh...and a rod bearing clearance big enough to throw a cat thru. Knocks like hell. I've still gotta get it home, but I'm gonna do that Saturday, I think. Then I can start partin it out. The big block will prolly get rebuilt and eventually go into my 85 Monte Carlo SS to try to break 200 mph in a street-legal car on the salt flats. I know it's been done before...but I've never done it.
Ohh...I picked the 74 up for $550. I figger I'll get at least that much out of it in parts...let alone the motor and trans.