The stamped number on the machined pad just behind the distributor will have the engine displacement as the first 3 digits if it's actually a GMC engine. (228, 270, 302, etc.) I believe the GMC's also have the two hold-down bolts through the center of the valve cover, like a 216 or early 235 Chevy, instead of the 4 small bolts around the perimeter that a late-model 235 has.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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