I appreciate your dilemma, john13. I would definitely plan on replacing both pump seals with the reportedly better seals from the 450 engine. You can't really 'tear down' the pump very far without damaging at least the outer seal.
You also might ask the machine shop how much material they had to remove to flatten the head; that could give you a clue as to whether the head was the culprit. It is a known trouble spot, as are the pump seals (to a lesser degree). I think the crankcase seam - while a potential leaky area - is much lower probability, based on reports in this forum.
Since I know my head had low spots and obvious leak pathways, I am going to leave my cases intact for now. If I were in your situation, I'd do the same. It is not so much work to R&R the engine, in comparison to the time (and risks) of complete engine disassembly and reassembly.
You also might ask the machine shop how much material they had to remove to flatten the head; that could give you a clue as to whether the head was the culprit. It is a known trouble spot, as are the pump seals (to a lesser degree). I think the crankcase seam - while a potential leaky area - is much lower probability, based on reports in this forum.
Since I know my head had low spots and obvious leak pathways, I am going to leave my cases intact for now. If I were in your situation, I'd do the same. It is not so much work to R&R the engine, in comparison to the time (and risks) of complete engine disassembly and reassembly.