I am running a used D&D T56, was told it was a VIPER version, it is 4-5 years old. I question it if is a VIPER since it has 10 Spline Input shaft. Was told back then they had 10s. Anyway.....
I agree with Meat, 26 spline is the way to go for this power level and now my weak link. I am pushing 570ish to the rear. I have a solid axle so no 1/2 shaft worries here. You got a GT so no issue for you.
I will say I broke more 1/2 shafts at 450-560RWHP than I did T45s. T45 hung in there pretty good. She was still solid when I did the swap and sold her, but she was my 2nd T45 tranny.