Don't even think about installing the cam without the gear pressed on!

This is a strange one..... it's normally not that hard. I'm starting to think you may have a badly machined gear.

If it was a new cam I'd be thinking that your old cam had undersized journals and oversized bearings in the block, but that can't be the case since you are reinstalling the same cam.

So lets just confirm....

Oil pump and distributor are out.
Lifters are out.
Retaining plate is between cam and gear.
Retaining plate screws aren't sitting in the block.

If all that is done, the cam should go in.....

If you have a crank gear puller, pull it and see if the cam goes in without it in place (and also then check the cam gear/crank gear meshing out of the engine).

It's looking a lot like a bad cam gear to me...