Re: 304 conversion to 345 question.
The 304 and 345 share the same bore diameter, but the 345 has a longer stroke. The block deck height of the 345 is greater, which is why the intake manifold is not interchangeable. Things like the oil pump, flywheel, valve covers, etc. should interchange. Probably the pistons as well. The heads are different (smaller valves, etc on the 304 heads), but you may be able to put 304 heads on a 345 if you really want to. Crank is obviously different, as well as push rods, valley pan. Not sure about piston rods.
Hope that helps.