For a single the old Ford T4 will work for about 450 HP. The TP-38 used from 97ish to 99 is good for about 500-550 and the 99-02 GTP38 is good for about 550 HP as well. Has been done several times with good results.
Like stated, the T3's need to match on the turbine to make sure EG pressure stays constant on both banks. Tuning will be inpossible otherwise.
I have seen a 350 with one of the .63 T3s on it. It was in a van, and was just used for low rpm grunt. Remember the 60 trim T3 is good for 325 HP. It would probably make around 500 lb/ft of torque at 3000 RPM as well, so it would be a real stump puller with spool up instantaniously. Mighty be cool for a off roader or something, but on a street car it would be real restrictive.