The Two cruise servos: the ‘70’s (mostly metal) and the ‘80’s (mostly plastic) will interchange - but the fender bracket is different between the two. Everything else will interchange except differences between the years with switches and stop light switches.
The plastic version has more “steps” to it, so when you push button to speed up/slow down, it will do so in smaller increments than the metal version servo does. Also, the vacuum pod part is larger, mainly for smaller engines (4-cylinders) and will make a speed change faster than the metal version will.
I installed cruise onto my ’67 Plymouth Satellite using the plastic version servo (4-cyl version). The car was automatic 340 4-bbl. Driving down an interstate someone pulled from on ramp right into my path. I quickly changed lanes – so no problem. No cars beside me and I was alert to the possible conflict.
What scared the bejeevers out of me was the cruise control put car into passing gear and opened the 4-bbl. I was not expecting that, as the car literally took off without me doing a thing. The lane change didn’t slow the car down that much (I thought).
The fender servo bracket is an FMJ-body only part. There is one version for all metal design (early) and plastic version (late).
I attached (below) the diagnostic flow chart from my ’77 FSM (Factory Service Manual) – which should be the same up to ’89. If any differences, it will be minor.
This bracket is a 4-bbl (only) part and not so easy to find. The 2-bbl bracket is “L” shaped.
BudW