What is the compression height of your pistons, or the part number?
I have a set of SRP pistons I plan to use in a Magnum block. I think they are part number 34796 with 1.670” compression height. We still had room to deck the block to get close to zero deck with these.
Search for threads on four speed swaps with Magnum 360 engines. I know there was few asking about that. As long as you are not using factory Magnum fuel injection a four speed swap should not be too bad.