Ladies and Germs: It has come to my attention that some facts about GMC truck engines may in dispute here and I have stopped by to clear the air and set forth factual information, backed up by original factory documentation to any non-believers.
GMC offered the following V8 engines from 1955 to 1959:
288 cu. in., 316 cu. in., 324 cu. in., 336 cu. in., 347 cu. in., and 370 cu. in.
A list of corresponding applications (models) for these engines may be found on my website under "Serial Numbers" 1955 to 1959.
I have no documentation to state the origins of these motors such as whether they were adopted Pontiac, Buick, or other GM engines, but I do have the original factory lists of parts for each of these engines in their respective applications.
Since you're on the subject of oddball engines, why is no one discussing the GMC 702 V12 here? Ah HA! Now there's a very different motor for sure. There were also 305, 351, and 401 GMC V6 engines, and you can deduce that the 702 was a "fused pair" of 351's. Jolly Goodfellow's 6066 GMC site talks extensively about the rare 702 and he has at least two of those engines as I recall. If all this is a bit overwhelming to digest, try evaluating the GMC straight six cylinder engines from 1939 to 1959 for a real eye opener in numbers, configurations, and engine sizes!
Additionally, I have some Chevy data which I don't generally admit to since I am really a died in the wool GMC man, but...
In that Chevy data, I find the 1958 Chevrolet "322 cubic inch Loadmaster V8" engine used in schoolbus chassis number 10802 ONLY, a 54-60 pupil chassis.
While these engines are certainly obscure, they absolpositutely did exist and they absolpositutely were put into the applications indicated above.
In fact, since I don't publish Chevy stuff on my GMC site, here's the Chevy proof for any non-believers:
For the other listing, you'll need to poke around my site some.
Never start a pissin' contest with a skunk.