best replacement tranny

Best bang for the buck is a Tremec TKO 500 or 600 at about $1600. Cheaper than a T56 by $1000, and just about as strong.
 
T-56 is heavier, takes a little more horsepower to turn, more work to install, and costs more. It also requires more rear gear because the ratios are taller. That makes your driveshaft balance more critical, becaue the driveshaft spins faster for the same vehicle speed when you use steeper gears.

All that being said, I beleive it will handle more torque. They don't put (a version of) it in Viper and Cobra for nothing. So how much power are you planning for later down the line?