Left rear sags 68 coupe

Hi All, I have owned this car for a few years, last month I changed the wheels for a wider and slightly taller set. First time out I noticed it was rubbing over bumps, but only on the left rear. That's when I realized for the first time that the left rear was lower that the right, by about an inch.

Having had this problem years ago with a Buick, the first thing I did was put the rear up on stands and measured the front, but there was no difference there. With the Buick after stuffing around trying to find the problem at the back found it had two different front coils, the stiffer one was pushing the opposite corner down.

But that wasn't the case with my Mustang. The front springs were changed a couple of years ago for a lowered set, but I didn't change the rears because they had sagged enough the sit the car level - from the side anyway.

So I got a new set of lowered rears and had a suspension shop fit for them for me because I just didn't have time. When I went collect the car, it was outside in the middle of a downpour and I didn't have a chance to look at it, but noticed straight away on the way home it was rubbing but still only on one side. Sure enough, it still has the lean to the left.

I called the suspension shop, and they have suggested removing one spring and cold re-arching it to fix the problem. There will be a cost ($230) for this procedure, but they say no charge for r&r and wheel alignment. It occurred to me it would be cheaper to buy a set of lowering blocks and only fit one side.

I have done a bit of searching on this site and other places, and found a few others who have had the same problem, but couldn't find any posts with a solution.

Anyone able to offer advice?

thanks

Dave