I suspect the 300 ft. lb. rating for the T5 is a bit optimistic. The SM420s were only rated for 200 ft. lbs and prior to '67 they used them behind 283s, 327s, 348s and 322 Buicks in heavier chev trucks. Just sayin'. Though 5-speed Spicers an option and maybe standard on the 409s By '66 there was the BBC 366, 396 and 427 and in 68 GM introduced the SM465. 420s, apparently would take abuse but not from a 427 in a C60.
292s were introduced in 63 and I'm not sure about a 292 in front of the T5. They were all about torque and there are 3 firing pulses per revolution not four as with the V8s. I suppose in a half-ton pickup that might be OK but I wouldn't tow much with it.
