Slant six noob

Welcome!!
use the search on this site-- many of your problems have been asked before.

Your heating problem is very common to these old cars:
1. first get an after market gauge so you know the temp of the engine
2. you say your engine spews after a short hi way drive? check the "T" stat for correct operation. (simple way is to see if coolant is moving in the radiator when car is at operating temp)
I would flush the cooling system-- your car probably needs this anyway --
do this before putting in a new "t" (so as not to clog it) or water pump-- do a search many have ways to flush the system at home.

Post up the pics & have fun ,Lawrence