The T56 Magnum is amazing, being totally honest with you, its the best choice overall, though you are getting close to the 700 lb-ft rating. None of the 5-speeds are going to shift that nice. Even the tremec guys admitted it.

I have the 2.97:1/ 0.63 6th version with a 3.23 gear and 275-35-18 which is a 25.6" tall tire. 1900 rpm at 70 mph in 6th. With a stock stroke 340 and MPFI (with a 227/231 duration at 0.050" lift hydraulic roller) it will pull a hill at 1300 rpm. Fuel control is really important for being able to pull low RPMs.

I can assure you it shifts amazing at 6500 rpm. There is no comparison between its performance and an A833.