First of all, but some plate steel and make your own patches. I bought enough for several repairs for $5. The kits are a waste of money and the holes are not that accurate. Then, it's easy to align the repair plate by putting it in place with your new 1.5" hole lined up over the rotted area. I did it by eye, sighting on the bolt hole, then had the plate welded.Once I had the desired alignment, I let the body back down so the repair plate was held in place while welding. I did this for all four rear mounts.
69 RS Conv,355,M20,4.10's and I don't worry about stone chips.