Ok, this is going to get long-winded so pull up a chair.
First thing your coach was not built on a commercial chassis. It was built on a sedan chassis according to the VIN number.
Anyway, back to the matter at hand.
GM had discontinued the radiator with no part number in the catalog. Your car was built with GM RPO(regular production option) KC4, engine oil cooler and KD1 transmission cooler (auxillary in addition to the radiator integrated cooler).
The only radiator GM shows a listing for is for cars with KC4 and VO8, the last one being heavyt duty cooling. Your RPO list does not indicate your car is so equipped. The radiator they show for KC4 and VO8 is sized at 30 5/16" wide, 17 1/8" tall with a core width of 7/8".
After beating my head against the wall trying to find out anything more I called my contact at 1-800-RADIATOR. We use them as a alternate supplier when we need a lower priced part or like in this case when we cannot get one from GM. They are pretty much nationwide so they should be able to help you.
His listing, P/N 1517 is for cars equipped with the engine oil cooler and the previously mentioned transmission cooler options. And the same exterior dimensions EXCEPT for the core width, his is 1 3/8" wide. His part is aluminum with plastic tanks, IIRC your O.E. was the same thing but time dims memory.
So, FWIW you might try that, or if you need it I can get you the GM number but I'm not sure how the V08 cooling would screw up your mounting.
So, I'm not much help