Need some Help

Over the weekend when I was heading back from my girls house my truck started overheating and some how got water with the oil. I am thinking that it is a cracked head, screwed up intake manifold, or a bad head seal and since the engine is a 305 with a 160k miles on it me and some buddys were talking and thinking that I should swap a 350. How hard would it be to swap in a 350 it is a 98 chevy 1500 I know that I would have to get a computer for it but my question is would it pretty much be just remove one and replace it? Right now it is sitting at a shop right now in the town were it broke down at so I am not sure what the problem is other than that there is water in the oil and that a 305 will only last about 160k miles so. Any input is greatly appreciated I am going to be looking at gettin an engine from summit or jegs or someplace like that depending on what the shop says about it I am suppose to talk to them tomorrow.