A stock factory 10.5 used to last in a factory 340 for thousands of miles. I swapped a 340 into my 70RR and when discharged in San Diego from the Navy, used it to tow (itself loaded) a 70 Cuda on a trailer full of parts. I'm thinking you just need a heavier 10" setup. A factory 340 was rated at less than 300 and had gobs of torque.
A 10” or less 23 spline disc is almost unheard of these days. The flywheel is also not the greatest as they develop cracks at the PP bolts with the 10” clutch. This is a safer option and opens up a bunch of possibilities for other transmissions.
