My bad,, you're right, it does look like an 8 lug.
Wouldn't a 2500 8 lug have a GVW of ~8,600?
Anyway, 3.73 is too tall of gearing for a small block GMT400 that has to tow very much weight.
I would think somewhere around that range. I agree though 7.4 and 410. Especially for towing. 4wd trucks had 5.7 with 373. Mine is 5.7 with 342. But both applications wernt towing rigs. Just light duty... also that's 1500 not 2500. Having 2500 he should have a more beefier drive train