It depends on if you want synchros in all the gears or not. If you want synchros, The G-Force GF3650 will more than handle your horsepower. Otherwise the t56 Magnum.
If a face plated transmission without synchros is okay with you, I agree with Jeff that the liberty LSc5100 would be a good choice.
I'm not aware of any modern, high torque capable transmission that won't require cutting the cross member. Just save the pieces you cut out. They can always be welded back in if you ever want to.
The Liberty or the G-Force transmissions will hold what you have with the right clutch. They both use the Nash design that has been in use for many years and it's a race proven design that has been improved by both companies.
Like Joel said it will take some cut and fab work on the cross member with a custom built mount.
Gus
