I was just saying, if the motor overheats, the cap will start expelling as a possible sign of wrong thermostat. A small point.
The 1960 stock 305 V6 truck takes a 7 LB cap. If you go with a overflow tank, some systems need a zero lb cap, I believe, in order to expel and suck back in. Some of the overflow kits include a radiator cap. Don't quote me on this, just a possible warning about cap pressure.
I would inquire as to other opinions but using a 180 Tstat and 7 lb cap will work and is very close to original. That could be called matching, using the correct parts will make them "match" what was intended by GMC. If there was an overflow/recovery tank offered in 1960, maybe we can find what lb cap was used if you want to go that route. But if radiator level is kept down a little, it should never spill out the normal tube. If you fill to the brim, it will expel......and then seek it's own level probably. Just so the coils are covered. My too sense.
If you have a radiator correction lasting more than 4 hours, call your mechanic.
Looks like starting in 1963 the cap was 13LB (12lb-16lb rating/closed or open system). That must be when they started using factory coolant recovery.