Your concerns about getting too tall a R&P setup are not unfounded. Going too tall will cause you as many problems, if not more than going too short. It's the happy medium you're looking for, and unless you've got a dyno, or someone who is running the same setup, getting it right is basically a crap-shoot. That's why we all, at one time or another, do a little trial and error on our own, after we've crunched the numbers and limited the range of options. "Experience is the best teacher, but it gives the test first and the lesson after."
If your truck was running basically the same configuration of engine, gearing and tire size you could get the same final results as mine. I've discovered with the 235, that unless you go with a fairly radical cam (which I haven't) that the torque band is strongest between about 1900 and 2400 rpms give or take a 100 rpms. If you wanted to match your gearing to get the highest vacuum reading at a specific cruise speed, you would want your engine to be turning in the 2100-2200 rpm range. On my setup, I'm just a hundred or so rpms below that mark at 70 mph. But as luck (?) would have it....I've found that I'd rather cruise at a lower speed, so I'm going to go with a shorter gear. This would let me max my vacuum at a lower ground speed.
So did I calculate this R&P change? Nope, I just increased my ground speed, watched my vacuum come up higher at 75-80 mph, and recognized that a shorter gear, which in my case would be a 3.55 or 3.73, would get me into the "sweet spot" of my engine at the mph I most wanted for cruising. The Camaro diff, which I've found is plentiful, reasonably priced and adaptable (62" wide) in a 5 lug pattern, matches my current rim bolt patterns and will give me the gearing I need. The S10 Blazer 4wd diff is said to be the correct width and it may be available in the ratios you need. Granted, there are some mods that have to be made but I know some of the guys are running them. Hopefully, someone will pipe in and confirm that a 3.08 will put you at 2100-2200 at your chosen favorite cruise speed.
You mentioned earlier that you were running a 15" rim with a 24-25" tall tire in a 75 series....you need to check that, as that would have you running a very skinny tire. I've found that the 235/75/15 at 28+" tall, fills the wheel well nicely on a 7" rim.
As I mentioned before, you've just got to carefully gather your information, sort out which will work for you, access your skills for doing the work yourself.....and most importantly, check that bucket to make sure you've got the backup money to get the job done correctly. I hope this helps.
Dave
