I haven't done much research on the GMC engines. Right now I'm working on a way to replace the long-stem Chevy stovebolt intake valves with ones for a Pontiac 455 V8, by extending the length of the stems with a screw-on extension. From 1950 to 53, Chevy engines used with Powerglide automatic transmissions had 1.94" diameter intake valves. Those valves can be fitted to any Chevy stovebolt head, but they're getting VERY hard to find. The Pontiac uses a 1.96" valve with the same stem diameter, but the stem is too short. I'm working on adapting the stem length by threading the top of the stem and fabricating an extension made of stainless steel, attached permanently with red LocTite. I'm still in the development stages right now, but I'm sure something similar could be accomplished with GMC valves- - - - -probably easier, since those engines don't use long-stem intakes. Find a valve with a similar head diameter, stem diameter, and stem length, and adapt it as necessary. I've been doing similar stuff on round track racing engines for 50+ years.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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