If you have the funds I have a great idea..
How about a TKO 5 speed and a gear vendors overdrive?
I didn't even know they make them to fit on manual transmissions. the 5 speed will be a lot stronger than the 6 speed and the OD will get you the milage.
You will also have more gear selections.. This is what I am thinking of doing on the 68 since the torque of the big block would beat a T56 to death (and I already own a gear vendors unit).
Might be a good option if you are going to be making lots of hp/tq..
For that matter you could do a T10 (inexpensive) and a gear vendors.. od would be .78 i think.. dont quote me though
Or if you like you M20 you could hook a GV to it as well.. I think they are good for 1100hp and pretty much never brake. You would need to shorten the driveshaft though.. I know the unit is a bit price but it is an option and it also has almost zero drivetrain loss (though it does ad weight)