Jon I have to disagree
Pontiac went to the 242 first 3 digits only in 1966 and up for the GTO line.
1965 was a different story GTO was a option on Lemans line so every 65 GTO started with 237... I have had several.....
So the only way to tell is get that vin number to PHS and let that determine it
I have heard that a 5N on the data plate from the firewall is a code that determines GTO option as well but I would not trust that
petaluma the PHS will cost you $35 for a mailed version or if you want it quick they will fax it back within 24hours for another $10