Here ya go:
(Click the pictures for the giant versions)


The frame width is the same, the older frames have the fold over on the front there, preventing you from sliding the hitch in from the front. And no, trying to angle it in there, just doesn't fly, I tried for 1/2hour and 2 beers later, I still couldn't get it in.
But if you simply cut the front fold off (I added some guide lines in the second pic), you can slide it in. You could leave those off, or weld them back on if you want.
I believe 81-87 frames don't have the front folds. Someone with an 81-87 could confirm. Towards the bottom of the front fold on our frame ears there is a hole for a bumper bolt, you can leave that part of the fold, you just need to cut the height of the hitch plate off to be able to slide it in.
I never checked if the holes line up, but you could drill new holes in the hitch or the frame if need be.
I would think, one side would be enough to get it in there too. You might have to do both, but once you cut one off, you could try and weasel it in there, and you might be able to.