261 is the logical choice and still resemble original.
A GMC 302 can also fit with some radiator adjustments. The front mount crossmember is different between GMC and Chevy, so could swap it, or modify/adapt.

Have you checked lift on the cam?
I'd check everything that's easy to check before pulling the head, might not be any trouble under the head.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-