For the current post 64 Classic M70 actions, my understanding is that the "long action" and magnum actions" are the same length / foot print. The differences that I am aware of are:
1) Shorter bolt stop and ejector. Allows the bolt to cycle H&H length cartridges further to the rear to clear the front receiver bridge when ejecting loaded cartridges.
2) Magazine box does NOT have the 0.200" spacer welded to the rear of the box to limit COAL to 3.400". Likewise a 0.200" longer magazine follower.
3) The one outward visible difference is that the front of the rear receiver bridge is milled further to the rear. Hence the shorter scope base mounting screw spacing. referenced in the OP.
It is possible to convert a standard long action (eg: 7mm Rem mag) to an H&H length cartridge. All that needs to change are the bolt stop, ejector, mag box and follower. While cartridges may occasionally contact the front of the rear receiver bridge during ejection, I did not have any issues as a result for the one rifle that I had converted.
Having said all of that, for the novice, it is a lot simpler to just start off with a rifle that left the factory as an H&H length cartridge. eg: 7 STW or 300 Wby. I had to play with 2 or 3 aftermarket long magnum magazine followers to ensure 100% consistent feeding from the magazine.
