The big problem is that the 80's grenadas are Fox bodied cars and have strut front suspensions and 4 link rears. You *could* use it all. Fat Man Fabrications makes a strut conversion kit for the early Mustangs and Falcons. And if you have access to a welder you can put spring perches on the rearend. However this will be the smaller 4 bolt pattern (4 on 4.25 " vs the later 4 on 4.5") so your stock rims wont work and the fatman kit is spendy.
If the engine/trans is the same type as you already have in your '65 small 6 with auto trans, it will be drop in ordeal. If it is different C3/C5/FMX (I dont know when they were used) you may have to do some modifying on the driveline, linkages, hose routing.
anything can be done with the right amount of time and money
-ron