Do you need to still do the 55 degree holes for a crossover in the Siamese block like the R3 block? Also should you still plug the rear port on the block with the 1/8" hole in the plug?

you probly need to ryan @ shady dell speed shop,he'd be the one to ask,if anyone knows it will be ryan

I am no authority , but I thought the large holes on the end of each deck weren't on the magnum blocks ?
I did plug and restrict them on my R-3 , but I am not doing the cross overs.
This is especially important at the front of the block when you use conventional cooling system , as it allows the coolant to go into the block from the pump and pick up more heat than if it were allowed to go right into the head and out.
I guess if it acts hot blooded , I will have to address it , but I plan on E-85 keeping it cooler than race gas.
That and a big radiator , plus plumbing coolant in through the center of the block , then up to the heads and out should work I hope.
I am still on the fence about drilling a passage between the exhaust seats on the center of the head , but this is a W-9 deal and I don't know if it applies to the magnum.

Yea, supposedly the magnum gasket covers the front hole. I am also going to use W9 heads.