Leveling the rear with lowering blocks?

Get the frame alignment checked.

I wouldnt suggest that. It will only add to the geometry being off. Check a few things for damage. Somebody may have replaced the rear frame rails and did a poor job or It could have been rearended at one time so check your rear frame rails for damage (or even rust). If these are old leaf srings (I'm not sure if you were saying your springs are new too).You could also have a worn leaf spring bushing on one side or even a weekend spring itself.

I did exactly what you're thinking of several years ago with my '65 fastback. After swapping in new springs and even a new rear axle, it still sat lop-sided. So I added a 1 inch lowering block to the passenger side and BINGO! problem solved. No the world didn't come to an end because of it, the car didn't ride or handle any differently and nobody ever noticed, so I guess it was an acceptable fix. Why did it sit lower on one side to start with? Who knows? Maybe because it was a 25 year old car that had been through countless owners who may not have been gentle with it. It had never been wrecked, but jeez how long can an old unibody car last without sagging a bit? How long does a human body last without sagging? If you can't find any damage to the frame rails (front or rear) and the springs are new all around, I wouldn't think twice about adding a block to one side. Ever see what circle track racers do to springs? They run springs stiff enough to hold up a dump truck on one side and extremely soft springs on the other. The use wheel spacers on only one corner to get what they want. They run more caster and camber on one side than the other, longer A arms on one side, offset the rear axle and then when they're done they use jacking screws to put pre-load in the corner they want. Don't listen to anyone who tells you your car wil be unsafe by adding a little more of a spacer on one side than the other.

I found out that the body was hung crooked on the frame. That was due to crash damage from a PO. Before I had it fixed, one side was lower and the bumper did not sit straignt on the body. But now it is all good.

I have no issue with shimming it, just a different way from what I did.