Ditto to the above... I picked up a core from a '71 motor home for $125.... Had a friend rebuild it using a Jakes full race kit.<$400 for kit. He went through it for 350. That adds up to $875 for a STOUT trans I will never break. It is going behind a 509 BBC with a Gear Vendors.
I just checked Jakes site and his prices have gone up a bit. $450 for the Stage II TH400 kit. Still a cheap way to go for the quality of the parts AND knowing what you are getting...
Kits and Components
There is also a lot of info here if you look around in the tech pages.
You will want to spend good money on a convertor also. Talk to a manufacturer, They will need ALL of your vehicle specs.
P.S. I have a beautiful smooth drum if you need it. Jakes is getting 120 for them. I will sell mine for 90 + shipping...