The '54 235 is unique in some ways, and unless you are willing to make modifications there is no newer motor that will fit in your truck.
However the 235 was in general use through 1962 and with some work, like changing the water pump and maybe in some cases the engine mount, the later motors can be swapped in.
Other options include the Chevy 261 and the GMC 228, 248, 270 and 302. I've read that the 261 was used in Canadian Pontiacs after 1962, so that may be the correct answer, but I don't know how accurate it is.
The motors I've listed, with the possible exception of the 228, were all offered with the same Hydra-Matic you have. I personally own a '54 GMC with a 248 and Hyrda-Matic.
