Jerry,
So far every 2.7 I can come across used, has a 6500 gvw, with 3300 springs etc on the front and rear. Yes I know for ALL that have them, them 3.5 has mucho power. I drive a detuned on at work in a 250 transit van. I know of no one that gets above 18 mpg with it! The 3.5's or 5.0 motors do have higher spring packs. For what I do, I dont really need more than 300hp, I'll take a bit underpowered, go a bit slower loaded, and get low 20s unloaded, I'll be a happy camper vs the 11-13 I get now with current truck. ALtho new, I did get 17 on the freeway a few times. Most of these heavier 15's should carry a ton with out too many worries. If I need more, I have my IHC which can carry 6 happy, 7 not as happy, 8 becomes squirrelly and I'm over my registered 26K gvw.
marty