I guess this really hasn't been done/talked about enough, but if you are going to cut the floor up anyway for the Keisler unit... Tremec T-56 6 speeds out of GM f body cars can be had on E-bay for 600 bucks. Quicktime makes the bellhousing to bolt this trans to small or big block mopars.
Doesn't take much reading on ls1tech.com or other GM/Viper/Cobra sites to see the t-56 has been proven FAR past its given torque rating.
As far as 5 speeds, there is a lot more market for a $5k bolt-in trans than a $5k cut-it-up trans.