Heater Box retainer Clip reinstall Help

Hi everyone. My first time EVER starting a thread on any forum, so here I go.
I have an '72 Dodge Dart swinger, 318 engine with A/C. I'm replacing the heater core and there are 11 retaining clips that hold the two sides of the housing box together. My question is. Is there a trick to reattaching the retaining clips or is there a special tool? I removed them with a screw driver but when I try the reverse procedure of inserting the screw driver into the slot and prying upward , or prying downward and pressing in the center of the clip hoping to slide it over the lip of the slot, there is no relaxing of the tension on the clip which allows me to lift it over the lip and snap it into place. I'm afraid if I pry too hard I will break the fiberglass slots. Each time it slips off the screw driver and simply becomes a projectile. Any information will be greatly appreciated.