With the 1" Shelby drop, the deal is to make sure the new holes are the right distance apart or the arms won't fit. The 65-66 cars are 3-3/4" apart, the 67-up cars are 4-3/4" apart. I start with a punch, then go to a 1/8" drill bit. Three more steps of drill bits to 1/2" measuring each time. I finish with a 1/2" bit and dress it to 17/32 with round file.
I have paper templates that are the right size for both 65-66 cars and 67-up cars. They are free, including shipping in all 50 states by sending an email to opentracker@sbcglobal.net -
I'd measure it and maybe compare it to the templates found in catelogues, like Branda. Some printers will alter the size of a print-out a little. I wouldn't trust that it's correct without checking.
It printed 1/16" off with our printer. It is a good template if the holes are made 3 3/4" apart for the early cars and 4 3/4" apart for the 67-up cars. Even with my metal template that I have been using for years, I always check the measurement. Measure twice - drill once.