Heads selection help 5.9 magnum

Speedmaster does not make a good Magnum head. You can put LA style heads, like the Speedaster LA aluminum or others, onto a Magnum block and run LA style intake and valve covers, but you will need LA rocker arms capable of oiling through the pushrods, like PRW stainless rockers or the Comp Cams stainless rockers those PRW were modeled after.
The Edelbrock Magnum aluminum head is a good one, but you still need new Chevy style rocker arms to run them.