I have a couple of body mount holes that need repairing
I plan on using https://www.opgi.com/chassis-suspens...it-c21159.html
What is the best way to get them in the exact location for the original ones are rotted/worn out?

this is a replacement frame that I am prepping to send to get blasted and powder coated.

 

I had to repair 6-7 of the Frame mount holes (72 Cutlass Convertible) after attempting to use Large washers, similar to what you have shown,
I ended up using Plate steel and just cutting out the frame and welding in new steel
I then used a Frame Schematic and measured the Mounting locations and drilled new holes ( measure 10X, cut once )
The Body lined back up with the Mounting locations perfectly

 

I’m sure you got this by now but I got some washers from fatenal. The ones I used are I. My mud thread in major projects. Hella cheaper.