I've never bought a transmission before...

whatever you do,dont fall for any " deals " on " just rebuilt transmissions " from anyone whos not a builder or shop.
an option like ocdart suggests would be preferable.
also they will want to know some basics of your motor build,
compression and cam and rearend gear ratio to match the convertor.

also,youve just dropped 450 horse into your former smallblock v8 car.
that power/torque is going to find every weak spot....
suspension,u-joints,brakes,springs rearend,etc etc...plan accordingly.

ps,ocdart,what did your trans cost when all in and all done??