Bent Tstat is very often a problem w/ heater core or plumbing.
Tstat closed then most or all coolant flow is thru the heater loop.
Bad core etc can cause big headaches for most cars but Fiero and a few others can bend Tstat "handle" to release pressure from the Water pump.
Very common to screw up heater plumbing for engine swaps too.
See my Cave,
Heater and
Thermostat------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave