Power transmission products are almost universally rated by input torque; and Tremec's specs are generally very conservative. For example the 3650 is rated for 360 lb-ft, however people routinely put 450 to 550 (even 600) lb-ft through them.
I would not be concerned about applying 740 lb-ft to a T-56.
FWIW it's the 1st gear ratio that most affects input torque capacity. If you look at the specs for the TKO series
you will see that those with the 3.27:1 1st gear are rated for 500 lb-ft, yet those with the 2.87 low gear are rated for 600 lb-ft; the only significant difference being the 1st gear ratio.
The T-56 used to be rated similarly (and very conservatively) at 600 and 650, for the 2.97 and 2.66 1st gears. However since they re-did their website they are all rated for 700 lb-ft.
A 3650 with a 2.87 1st gear (something I would like to have) would likely be rated for 430 lb-ft...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq
Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.