Murph,
Since nearly every thing else has been mentioned already, I'll relate an experience I had. My Cub (150 narrow deck) ran really warm, even in Alaska, mostly oil temps, but also cht. We checked all the stuff mentioned here, nada.
I finally called Lycoming. Bob Ohnmeiss told me to get an automotive compression tester, and hook it up to each cylinder, and do a compression test while motoring the engine through with the starter.
The automotive tester (as opposed to the aviation compression tester) is designed to determine if a cylinder is MAKING compression, not whether it will hold pressure in a leakdown test.
Long story short, I had two cylinders with bad rings (one with broken ring) and a hole in one piston. Engine was running hot oil temps due to blowby into the case from the combustion chamber.
Might be worth borrowing an automotive tester for a quick check.
If you have hot oil temps, I would also check the gauge, as noted above, EVEN though you just had it rebuilt. I recently bought a NEW tachometer, installed it, and it was 100 rpm off. Manufacturer's response was that was within limits. Not for me. Digital tach now, even though I don't like the things.
Finally, the Cub has a pressure cowl. The theory is that ram air goes in the top, and is forced by baffles through the cylinder fins, and out the rear bottom outlet. Top is pressurized, bottom is exhausting. Put an opening (oil cooler, for example) in the bottom of your cowl and you pressurize the bottom of the cowl, which defeats the purpose of a pressure cowl.
Nonetheless, Cubs work fine, as noted by others, with the oil cooler mounted up front.
Have you checked all the lines, and double checked that the vernatherm is actually functioning correctly, and set right?
Good luck, and here's hoping you don't have the problem I had.
Mike Vivion