I don't think I understand your question. You said you got a BLOCK from a guy but the mains are snapped and razor thin so you wan't to know if you can use a different crank?????
To the second part of your question: Lots of folks swap cranks, rods, and pistons around without balancing. If all of the weights between the old and new parts were close and this isn't a high HP high RPM engine you might get away with it. IF the weights of the parts were way different you won't. Do you know what the crank donor engine had as far as rods and pistons? Had the rotating assembly been balanced for that engine?
Not trying to discourage you but there's not much information here and you're asking broad questions. If you can provide more information on what parts you have and what you wan't to do with them we can give you better answers.