this is the link to the Tremec site.
http://www.ttcautomotive.com/English/products/TKO.asp
The big different in the 500 & 600 beside the torque rating, is first gear, 3.27 vs 2.87 and the 5 th gear, .68, .64, .82.
I bought the 600 because I needed the 2.87 1 st gear.
The low end torque that "your" motor makes will make a different in the trannie you need. Tire size and rear end ratio also play a big part in the puzzle. I bought 3 trannie's and 3 different motor before I was happy with cruis'in in my Cobra.
1500 rpms at 50 mph in 5 th, no trailer hitch jerk, and 2000 rpms at 70 mph. I have enough torque to cruise in 5 th from 1500 rpms to ?????
Any gear is a passing gear
Dwight