Help needed with frame to cab mount brackets 69 cab on 65 frame

[tranz am]

I have been working on a custom frame for my 69 GMC short bed that you can see in my build thread in my signature. the frame that I've used is a 1965 c10 short bed. I was told the frames were the same 62-72 and they appear to be the same with only slight differences.
Anyways I have determined one of these differences is the rear cab mount bracket, which seems to be an inch taller than the one used on the 69. I had a spare frame from a 68 which we had previously cut up and i have the section where the rear cab mount bracket normally is plug welded to.
the problem that I am encountering now is that none of the holes seem to line up. There is a big hole that I believe to be a positioning hole. my question is do I use this hole to center my 69 cab mount bracket? here are some pics. Any insight with this is apreciated.