Hello, kclark2227
A 261 was an RPO (Regular Production Option) on 5000 and 6000 series 1954 Chevrolet trucks (a 235 was standard equipment). As posted above, a 261 was a desirable (and common) option on big trucks. For 1957 and later 5000 & 6000 series trucks, the 261 was standard equipment.
If you want to ID your head/block, find and post
the codes shown on this page.
If you swap-in a later non-54/55st 235/261 engine, you will want to install a water pump adapter that places a water pump in the correct location. Ask here for information on how to do that.
You have a 6.17:1 rear end ratio - that will significantly affect your top speed (with any engine).
