tremec tranny and 390 engine

Subscribing. I'd like to know as well. I have a 67 Fastback with a 428 and I want to swap to a TKO 600, but I want to be able to keep the console. Keisler sells a kit that they say requires "minor tunnel modification" but still allows you to keep the console if you hve one. Kinda high priced but might be worth it if it fits as claimed.

i have a c6 in mine with a 390 bigblock and id like to change to the 500

I too, have been looking into this. The problems seem to be keeping correct driveline angles.
As far as the Keisler kit, the transmission itself is slightly modified to fit (the shifter location and some grinding on the case), and the tunnel brace must have a good section cut out at the top.
I have also heard that the use of the TCP motor mounts that move the engine down about 1" and have about a 1/2 " of side/side, forward/backward adjustment also help in keeping the driveline angles correct. I have not heard of any other side effects from lowering the engine so these might also be a good option. Probably would be a lot stronger than the stock mounts also.
Any other input would be welcome as I am sure there are more than a few of us out there that would like to get some cruising mileage out of the ole big block.

These guys don't live too far from me. They should have everything you need or need to know.

http://www.keislerauto.com/ford/transmissions/mustang_65-70.asp