ryan,
If you are referring to a 54 235 truck engine, most parts are interchangeable with 54-62 235 truck engines and, to a slightly lesser extent, interchangeable with a 53-62 235 car engine. As 38 Canadian says, a 261 engine is very similar but there are some significant differences in comparison to a 235 (also, depending on the year).
If you find an engine, post its casting codes here and people will advise you about the differences.
If you are going the JB Weld route (temporarily, perhaps), clean the outside of the crack very thoroughly with a wire brush. At the ends of the crack, drill an indentation into the block. With a grinding point, groove out the crack. Then, apply the JB weld.
Maybe other people who have tried JB Weld on a small crack in the block will also offer advice (unfortunately, I had to do this about 12 years ago on a 261 block - it is still not leaking - I am lucky on this one).
