in my opinion 600 hp for sure is solid roller territory, the stock hydraulic roller setup is not usable - no advantage for the Magnum. I think either platform can be used, but honestly i think the Mag 5.9 blocks are available in better condition to start with. Valvetrain oiling has to be considered, but there are solutions to use mag heads on LA or LA heads on a magnum.

Regarding the intakes: there is a Indy 360-3M intake available with mag-style bolt pattern, so this is no deal breaker.

Michael