The 560 may be a little stronger, but its also a little heavier.
I would go through the records of the 560 and see if the chain was done, and if the valve seals were done. If those two things were not done on the 420, they will be needed. Check out the underside for any oil leaks/trails. See if the radiator was replaced on the 560-if it wasn't eyeball it closely. I think (not sure) the 560 is self-leveling in the rear. The 420 is not.
Both will suck gas. Check the radios - they came with a 10 speaker system, if its been replaced, make sure it was done right.
The 420 will need front end parts (nothing super expensive) the 560 should have already have them replaced.
In many ways, the repair records of the 560 will tell you what you are going to repair on the 420. If you can find an 89 or newer, you will find they have a differently stitched interior which, in my opinion, is nicer looking.
Good luck with your decision.
PS: I just saw the 94 Pathfinder reference. Your mpg will be about the same. ( I was getting around 16 on the highway) Btw, did your exhaust manifold leak too?
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