A 70s truck in the Midwest rust belt? Any seal along the firewall, cowl, roof, and vent boxes and any where 2 pieces of metal are spot welded together. Really!
Also around any and all of the glass, and door seals and where any wires, hoses or fittings enter the cab!

They were not very water tight back in the 70s, now they have had 40 or more years to deteriorate. Besides looking for where the water is coming in, you also want to be sure all the body drains are open so that whatever gets in can get back out. Besides the heater/AC box drains, there are body drains at the bottom of the cowl, bottom of the fenders, bottom of the doors, and bottom of the rockers, (I'm sure I'm missing someplace else). Open them up with a piece of wire.
Good luck. Gene