I've been through this a couple of times.
The 87 4.2l takes a certain starter.
Problem is the 4cyl starter (my guess) can be mistaken for the 6cyl starter easily.
The Zoo (AutoZone) has had it mixed up for years.
I had the manager convinced, but thier corporate management wouldn't listen to her.
To tell the difference, look at the aluminum housing OD and compare it to the OD of the metal case of the starter drive. If they are the exact same size, it's the wrong one. If the alum housing is slightly larger in OD than the metal case, it's the right one. It is a noticible difference in size, but not really that much.
If you have the right starter and it's still grinding, it's not aligned correctly.
Another commented above on pushing it close to the block.
It's true. I've had to do it many tiimes with 2 different transmissions.
I've been able to figure out the 87 flywheel is specific, but that's for a clutch.
I don't know about the flexplate on an automatic.