You probably did something to the engine when you changed the thermostat, like a damaged a wire or something. You might replace your spark plugs and wires if they are older.

So your old thermostat was stuck OPEN. Now since you "fixed" it, something else is wrong. Sounds like it's overheating. Did you put the new thermostat in upside down?

Maybe just coincidence, but I've seen problems with that engine and the EGR valve.

yes

I recently had the thermostat fail in my 94 GMC k1500 with 4.3L engine. (Engine was running cool and the heat would take forever and a day to start blowing warm.)

I replaced the thermostat with NAPAs recommended part and no the heat works and the engine temps are normal. But Instead of 19 or 20 MPG on the highway I am getting 16-17 and the truck has much less power.

Any ideas as to why or how I can fix it?