For anyone following this I spoke again today with the folks at Meziere. They reinforced the idea that I should not use the standard FE 42 gpm pump with an 800 HP engine, and that a 55 gpm pump was needed. They suggested an externally mounted pump, which they make available, but I told them I didn't have room for that. I also asked if a 55 gpm motor and impeller from another pump could be bolted into their FE housing, but again they said no, there were several internal differences in the housings and that this approach would just limit the flow rate of the pump.
Mario previously sent me a drawing for the BBC pump, and the water outlets of the pump were REALLY close to the FE water outlets, but unfortunately the water inlet is pointing the wrong way for my radiator. He also suggested that I just buy a gasket for a 385 Ford engine to see if it would fit. I don't know why I didn't think of that myself, but yesterday at lunch I took Mario's advice and went to the NAPA store to pick up the gasket. It was $3, and it looks like the 385 series pump has water outlets pretty close to the FE too. So I think I will proceed with that pump, since it is available with the water inlet pointing in the right direction.
I will post more info under this thread when I have acquired the pump and made the adapters required to bolt it to the FE block.