Not sure about '65 but I can say with authority that the squarebody rivets ('73-87) were not installed hot. They were cold headed.
Also - (again squarebody) a 3/8" grade 8 bolt was used on the assembly line as a repair for any mis hit or missing rivets.
I checked the generic info packet from the GM Heritage Center pgs 168-169 and it just says frames are "riveted" (does not specify hot vs cold, nor does it specify size)
https://www.gm.com/content/dam/compa...olet-Truck.pdf
Hope that helps or gives you some food for thought.
K