Any trick to get good heat out of 80 pickup truck?
I placed a piece of cardboard in front of rad and the
gage went up but the heat didn't.With the fan on defrost
I get cold air coming out the vent's

Make sure the little valve on the firwall that allows hot coolant into the heater core is working. If it is then maybe pull your heater core cause it is probably full or stuff (dust, dirt leaves etc).

I think it's working because the lines are hot going in and out.
Would the heater core need to be replaced or just cleaned out?

probably just cleaned out. Mine didn't really make any heat unless you were doing 3000rpm or higher.

The engine make good heat it's just not getting hot in the cab

Take the hoses off the heater core and reverse flush the sucker with a garden hose. I am not sure about the 1980 Toys, but the 85 and up are a PITA to remove the heater core.
As Shep said though, make sure all your cables are operating properly.
Good Luck,
Tye

I'll try that.
Thanks