If I may be so bold as to suggest, the M90 is too small in capacity for a 4.6 litre motor and barely capable of handling the 4.0 litre one. Does it come with an intercooler? If not, it will eventually destroy the engine due to pre-detonation (pinging) in the higher rev range, because without cooling, the intake air temperature would be too high. The ECU might handle the electrical side of things, but you would have to have had, a fuel controller of some sort. I really hate to throw cold water over your sale, but personally I think your asking price is way too high.
