No, the issue is that only the early Magnum blocks are still drilled for head oiling, a holdover from the LAs. Later blocks omitted the passages entirely, as Magnums oil through the lifters & pushrods. Confirm that the passages are there in your Magnum block and you'll be okay. The LA roller block is fine as-is. I'll see if I can find a pic.
And by the way, since you're swapping from a slant to an eight, don't forget to hunt down an early A V8 centerlink. And the V8 throttle pedal and linkage. And for both of the blocks I mentioned, the 360 (NOT 273/318/340) pass. car oil pan & pickup.
