not quite so 49er - water expands much before reaching boiling, and the overflow is there to prevent overpressure due to either overfilling [common] or overheating [unusual], and the coolant in a properly maintained [and filled] system will never use it [freezing will crack your block, extreme pressure from boiling will split the rad] - note the fact that there is a specified pressure for the cap and a specified temp for the thermostat AND the operators manual that came with all these ol trucks specifically states:

"volume in a Chevrolet cooling system expands about 1 quart when temp is changed from 32*F to 160*F -
therefor the solution should be left 1 pint to 1 quart low if filled cold to prevent loss of solution through the overflow pipe"
further on discussing antifreeze it specifically states coolant loss through expansion "is a result of overfilling"

also the standard thermostat is 160*F on all models, and the cap is 4# pressure on early trucks, 7# on later years
check your operators manual

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics