Rear engine power loss during flight

[mshac]

Your 1967 does not have the engine access panel in the rear bulkhead which allows access to the rear engine from inside the cabin. This leads me to suspect the mags even more, because without that access panel, the rear mags are very difficult to tighten properly. Use your hand and see if the mags are loose at all. Even if they seem tight, I would still check the mag timing if you don't find any other issues with the engine.