88 dakota gets hot

It's just an ebay 3 core aluminum it cools it great, doesn't have any leaks just can't keep up with the a/c goin . I just don't get it I had a 64 valiant that had a s6 in it. I took it out put a tunnel ram 340 in it left the little 2 core rad in it and didn't put a thermostat in it just to be sure I wouldn't have an overheat during break in and it ran so cool that I put a different carb on it thinking something was wrong with it cause it wouldn't idle and I put a 180 stat in it problem solved so why does this dakota run so hot ? I took out the stat and it still runs at 180 so I put it back in and it runs at that temp wether your pulling a load in july or cruising downhill in January and you can run thr a/c unless it's over 93ish and your in city traffic.