... There Rockwell axles, I assume??
You can get custom rims built (for alot less $$ than you think) and then run 15" tires......
But theres alot of those 49" Michlen ex military tires that are nice also that are ment to run on those rockwell sized rims......
I have heard good things about Boyce....
I've heard enough BAD things about a guy named Daniel from 6X6 USA (or somthing like that) to steer you away from him.
Another thing is that with those Rockwell axles you have to use a link type suspension if you want any type of suspension flex because leaf springs will have to be too stiff to stop the springs from wrapping and doing bad things.......
So, once you've built the cross menbers for the links to mount, you "might" as well keep whats left of the stock GMC frame......
Check out
http://www.pirate4x4.com/forum/ for more ideas, the search has been down for a while there otherwise I'd tell ya to search.........
And ......... what size tire are you thinking of??