The parts guys at Sterling GMC (top notch guys) figured it out today. Used a big block shroud and insulators. These radiators mount differently--they use metal brackets instead of just the plastic shroud.
The 350's fan doesn't reach all the way into the shroud but good enough I think. Might get a longer shaft fan clutch at a later date if it's a problem.
Now I only have one problem left ot solve: the radiator-ends of the engine oil cooler lines don't fit the big radiator. Kinda like one is metric and the other is std. Hmmm.
If you ever want to change your 28-1/4" radiator to a 34" just let me know. I have all the parts numbers. BTW, I spent $350 on a radiator and $100 on GM parts (shroud, insulators, brackets).
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'72 GMC 3/4 Longstep
'92 Chevy K1500 XCab LWB 350/3.73s
Aggieland, TX