The lever action cap also will NOT work. I always bought Champion aluminum radiators if I could. I had their 2 row replacement in my 413 CI SuperBee and it worked great. If I needed one fast locally, I bought Modine. The ones for my Fiero and one of my C4 vettes were identical...so if it can keep a 350 V8 cool, it should have no problem in any Fiero. All mine easily kept 160* temp no matter how hot it was outside.
Make sure the cap is Not "Touching" the hood rail when the hood is shut. I had that problem after I installed a new radiator, (a matter of about 1/8". had to cut a spot into the rail to relieve the pressure on the cap )
You can have problems using 3 core on 2.5l engine.
Most times the duke doesn't generate heat to make problems on OE rad.
You can use too much radiator for given engine and weather making the issue very hard to predict. If you cool too much, the engine gets problems cause slow to warm up or act like always trying to warmup. This can hurt MPG and Power. OE type Tstant will flood the engine w/ cold water. You often get see that happening on temp meter on the dash. When system is working right using a OE rad and driving in cold to freezing weather the temp will climb fast when Tstat is close. Tstat open and engine temp crashes. Each Cycle can be seconds to a 2-5 minutes depending weather etc. The giant radiators on big trucks are close off by covers over the grill etc in winter for this reason.
Stant SuperStant helps stabilize engine temp w/ OE rad but adding more cooling area on the rad then that might not help many 2.5 and some others.
On topic... Warning: Bad seal of tube cause by weak/damage clamp on either end or Damage to overflow outlet itself often behaves as a bad cap or area on rad meets the cap. I have Aftermarket rad w/ "flat" area on overflow outlet that fail to seal w/o carefully adjusting small worm screw type clamp. Small size for them makes "flat" area at the screw and when the two areas meet the hose can seal.