Welding rear axle spring pads for handling

Correct, the pre-'71 B-body rear axles with the perches in the stock location would spread the springs a 1/2" per side on an A body. And reduce tire clearance.

I've heard of guys slotting the center hole on the perches another 1/4" or so to limit some of the side loading on the springs. Of course then the spring doesn't sit centered on the perch. It just seems like a lot of work to do it wrong.

Moving the spring perches isn't that hard. It shouldn't even cost all that much if you can't do it yourself. Plus, if you're moving the perches already, it's a great time to do a 1/2" spring offset to maximize your tire clearance with the stock wheel tubs. That's what I did, my Duster has a 68-70 B-body 8 3/4 with a 1/2" offset, perches are 42" c-c.