Magnum head and exh manifold ID

I've found a 318 magnum powered van in a junkyard and wanted to check a couple of things:

do the 318 and 360 magnums use the same valve sizes and ports?

how much better are the mopar performance magnum r/t heads than the stock production ones?

i've read that some magnum exhaust manifolds are more desirable than others - how can i tell if this van has the good ones?

do all magnum powered vehicles use basically the same exhaust manifolds or are some a better fit in a bodies than others?

how can i tell what year the van is?

thanks