Yes, I can help. Even though my truck is a '52 GMC, I have done the wheel and tire upgrade on my truck, using 8-lug year 2000 GMC chrome 16" wheels. The bolt pattern was the same, but I did need to have 1 1/4" spacers made for them. The tire shop I bought the wheels from was able to provide me with the spacers the same morning. The spacers are made to bolt onto the original hubs, and then they have their own set of studs that the wheel then fits over with lug nuts. It works very well and I have had no problem with them. I believe my tires are modern Uniroyal all-season steel belted radials in 235/70R-16 size, and they handle at least twice as good as the no-longer-available school bus tires on split rims that I used to have. It was an excellent upgrade.
About your taller gear ratio, it depends on your engine. The 3.08 will give you much better highway cruising, but if you have an inline six engine, it may take you all day to get up to highway speeds. That's obviously an exageration, but the point is, with enough torque, the 3.08 gears will feel fine. You may almost need big block torque to be able to turn those gears good enough at lower rpms.
I have 3.08 gears on a 1968 Firebird with a super torquey 465 cube Pontiac engine, and they feel very good on that car. But even with a small block Chevy, I don't think there would be enough natural torque to make the 3.08's feel good.
You might be better off compromising to a 3.55, or maybe even a 3.73. And my personal opinion is that considering the age of the rear diff, it is well worth it to have it gone through and rebuilt. If you trade for another old rear end to get the gear ratio, that diff too could require rebuilding, and then you're in it for more money. Better to do it right the first time.
