Hang in there on your GTO get all of the driving/handling things complete, then the tuneup perfected to where it will crank with no problem and cruise around town in the heat like we have been having without pushing water out or over heating.
When all of that has been perfected and if you like...
I could be wrong as I quite often can with my Wife but I think that the Tri-Power was offered in 64,65 and 66. I bought a one owner 67 in about 1969 or 70.
Yours may have been a 66!
I am a 66 year old Hotroddder, I still Drag Race three or four times per year, I run the Wanna Go Fast 1/2 Mile...