The 350 is a pretty obsolete transmission. It hasn't been installed in a new car since the middle 1980's. I'd expect to pay no more than $100.00 for a "rebuildable core" transmission, and certainly less than $500.00 for a rebuilt one from a transmission shop. Chances are there's a local transmission builder who would be glad to give you a bargain price on one that's been cluttering up his storage area for a while. Don't get sucker-baited by some hotrod shop that's selling a beaned up racing transmission that won't work well in a street application in the first place.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!