Everything but the block itself and the pistons/rings is exactly the same. The two blocks will both bolt in place of each other but the bore sizes are different. You can use the 305 heads on a 350 and it will increase the compression, but it will run out of breath at about 4500 rpm. Really need some better heads. Valve covers, timing cover/timing set, distributor, oil pan, oil pump, cam, lifters, headers or exhaust manifolds, and intake manifold, crankshaft, and rods all interchange with both engines. Should be able to use your rocker arms and pushrods from the 305 too.