Sounds like the A/C compressor may be contributing to the premature failure of the drive clutch. You mentioned and unusual amount of engine strain is/was noticeable when the A/C compressor was engaged, I am curious is the compressor noisy when it is engaged? After a couple of clutch failures on the same compressor I would be inclined to focus my attention on the compressor to determine if it is the cause of the failure. My .02


PS: I regret sacrificing my 1987 300 TDT, what a wagon.