Ted, Richmond 'conservatively' rates it at 450 ft lbs.
After re-reading it, they state "Capable of managing 450 ft. lbs. of torque at the rear wheels"
I wonder how that differs from the Buick 401 rating of 445 ft lbs torque??
I now have about 100 miles on my 401/5 speed combo. So far, so good! I only use 5th over 50 mph.
My GS's 2.93 rear/ 3.26 first is okay (9.55 overall) It's still a close-ratio trans, so expect to shift often. If you can afford it, a wide-ratio would work nice if you do a lot of Highway cruising at 70+ mph. I'll have more info after I get more miles on this beast.