I have two 1954 261 engines. One of them has a GMC oil pump, so that is not much help for this problem. The pump for my other one is similar to the the "original" in bowtietm's images. I'm pretty sure that the Melling is for a 55.2 or later 235-261, as the manuals and parts books show that the 1954-55.1 have the pressure regulator in the main pump body and the later pumps have the regulator in the cover. Check the Chevrolet Master parts catalog for the parts numbers. A 1954 (and probably a 1953 Powerglide) 235 pump should also fit your 1954 261.

Hoyt