Even if the 1957 235 needs repair, it will be a cheaper rebuild than the original one if it turns over. By the time you're done driving the stuck pistons out of the 1948 engine, you're looking at a rebore, and the lifters will probably need to be replaced, and the rusty crank journals ground.
The 1957 motor shouldn't be that hard to bolt in. Like Jerry said, the water pump would need an adapter plate.
Good luck on your project.
