First thing you should do is order the PHS Documents
and verify the 65 is an actual GTO. If your car was built in the Pontiac or Kansas City plant the data plate will include an accessory code for the GTO option.
The first numbers to verify and compare would be the date codes on the car, block, heads, transmission and rear end. All of the above will have casting numbers to verify they are correct for your car. The PHS report will specify which drive train is correct.
You will find codes on the front passenger side of the block, the distributor pad on the rear of the engine, on the center exhaust ports of the heads, just below the valve covers the heads will have date codes, the carb, the passenger side of the transmission, the tube between the differential and the passenger brake backing plate.