You are approaching this from the wrong angle. The 600 Holley is not too big; your engine is too small. wink

G.M.C. had a 503 CID 6 in the early 1950's. Change the engine, the 600 Holley ought to be fairly close wink Maybe just a little large for the 503!

All joking aside, Jerry has the right idea although I would probably suggest a Zenith from a G.M.C. 270 on your 250 and the original 250 manifold would be just right.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop