How much is a gm turbo 400 transmission worth?

Maybe $75 or so. Somewhat more if you have verifiable receipts for the alleged "rebuild", otherwise it's a pig in a poke. He's lying or seriously deluded about it being able to handle "800" horsepower - a TH400 can take maybe 500 max - so he's probably lying about the rebuild too.

Also, it it a Chevy or Buick-Olds-Pontiac bolt pattern (totally different and not interchangeable) and what length is the tailshaft (to know what vehicles it might actually fit)???

a TH400 is an old trans.....its pretty worthless as far as today standards go..a 4L80E is the modern electronic version of the 400 and is a 4 speed rather than 3 like the TH400....so Its only worth what someone will pay......which is probably not very much....