No, that's not true. The Magnum heads have 1.92/1.65 ports, the LA heads have 1.88/1.60 or 2.02/1.60 ports. "Out of the box" the Magnum heads flow better than the LA heads, I've heard of people putting Magnum heads on LA engines but never the other way around. Like you said, the Magnum heads need different rockers, pushrods and intakes but and also lifters. The bolt pattern is different so an LA intake won't work on Magnum heads and vice-versa. You can modify an LA intake so it will work but that's not an easy thing to do unless you know exactly what you're doing. I think that you'd also need to open up the oil passages on the Magnum to allow the shaft mount rockers to oil properly.The 360LA and Magnum heads have the same port size. The Magnum is the last evolution of the 360 LA head. The Magnum has a different rocker setup, oils rockers via pushrods (LA oils through block and heads). Intake mounts differently. No exhaust crossovers in Magnum heads. Despite the differences, if certain parts are swapped, the heads will interchange.
Melling oil pump. Meantime step up to 10W40, 20W50 in summer.