Cebus Capucinis

Gonna recommend the NP242, because a locking centre diff is *damned* useful and will handle a 350 no problem.  Can verify that it's absolutely usable with a manual transmission, despite what teh intarwebz will say.  Will require modifying driveshaft lengths, though.

That said, have you given thought to a 4.0 stroker?  Keep the bottom end of the 258, mate it to a 4.0 head, do a few other tweaks, and walk out with a 4.5-litre straight-6 with no need to get crazy with mounts, etc.  Not that those are a huge problem for a 350 swap - but I really love the I6

Hmm, looking at that I'd probably get enough of a HP increase for my needs. I don't need a 550HP / tons of torque beast because I'll never tow anything heavier than a small pop-up tent trailer with it.

I should also just jump the gun and put HEI in it, as that's a cheap gain of 20-30 usually.