hey Lee, how ya been? still lovin on those seats bud...
starting with the basics - don't trust the temp gauge. the terminal to the temp switch tends to stretch from being tugged on over the years. I've personally had it go from functioning normally to not, completely randomly but visibly; I looked at the terminal receptacle on the harness and had seen where it needed to be pinched to get consistent contact. Temp SWITCH, not temp sensor.
start with the hoses, as said earlier - when the car is hot (as it can get), car running crank the heat, check the heater hoses. with a new core, they should both be hot. both cold - check the heater control valve for the vacuum line from the firewall (and if it is connected, make sure it isn't fractured, can fracture ANY where - note valve actuator position, move temp to cool, position should change). if your rides aren't getting up to operating temp in the first place - could be too big of a bypass drilled into the t-stat, or sometimes you get a box from the parts stores that has been swapped, you need to check the stamping on the t-stat. I got slipped a 165' in a 180' box once...other than that, carefully check the rad/hose to see that they are hot.
the stock cores are slight, maybe 5/8" thick core if I remember. aftermarket replacements are thicker. I wouldn't do the core without the blower motor, original are both inferior to aftermarket as I've seen. if you didn't PROPERLY bleed the coolant when you installed, it can take quite a while to work all the air out before you'll get full coolant circulation (to the core). don't waste your time doing the CLR. I stretched my last core with CLR in extended soak, followed by water and air pressure clearing. barely improved. all else right in the coolant system, you should get enough heat off a new core to bake your feet pretty well. if not something is off, usually simple once you find it.