Gear Ratio and Tire Questions!

[tx72k15]

First off, I did a search and it didn't really return anything specific that I saw. Hopefully I'm not beating a dead horse but if I am someone please kindly direct me to a thread that will help. Thanks in advance.

Now for my questions:

I have a 1972 gmc 1500 mounted on a 1982 or 1983 suburban 4x4 frame. It's already body lifted 6" and has 35" Mickey Thompsons on it with about 50% tread left so I've been pricing new tires. I'd like to go up to 37" tires but I don't want to tear up my gears. I've always heard that 35" tires are as big as it's safe to go with stock gearing. Is this true? If so, what ratio would be safe for 37s?

Also, the tires I have are 35x12.5x15. I'm not exactly sure what width the rims are but how do you know what will fit? Is there some golden rule? If a 12.5 width 35" fits should a 14.5 width 37" fit?

Thanks for everyones patience with my rookie questions,

Doyle