I did the 7:31 CWP swap and wrote a long, detailed tech piece about it for Pelican that has yet to be posted.
The 7:31 is overkill for a 3.6 -- that engine produces so much torque that an 8:31 box would work fine. In fact, the 7:31 is almost overkill for my car, which is about 240 HP and 2,250 pounds.
The 7:31 is not "weaker" than the 8:31, it just has a force vector that is skewed more toward the diff cover, whereas the 8:31's force vector runs more along the axis of the pinion shaft. You should have an updated (post-'79) reinforced diff cover and a one-piece bearing retainer in your tranny.
Hmmm...rather than re-hashing all of this, I should just post the tech piece. Hey Charlie, if I send you the tech piece and the associated photos, will you post them?