I believe the Magnum's oil the rockers throught the pushrods, the non-Magnum's ( LA series....) oil the shafts throught the cylinder heads directly. There may also be an extra cylinder head bolt too. I know you can swap Magnum heads onto the old LA block, so it can't be much difference. I'd also check motor mount pad location.
BTW: I'm going through a living hell with a MP non-Magnum short block thats killing me.... It ate the cam in 20 miles and I'm on rebuild # 2. Have fun..... :salut: