assuming this is a 4.3 and if some of the bolt land is still on the outside of the block there might be something you can do.
(assuming you dont want to do the right thing an "bite the bullet" which would be best.)
it involves notching the block above the starter/ block mating surface and installing a threaded "plug" in the notch. it can be done in the truck but it would be difficult.
i have a freind that races a lot and he has succesfully made the repair several times.
in my experience that problem has always been caused by lack of support on the free end of the starter. usually the bracket is left off because of alignment problems when aftermarket parts are used.