273 4bbl, carb replacement?

I'd put a carb kit in the carb you have, but first check to see if the secondaries are fully open when the pedal is on the floor.
If you only have cooling problems at idle, you are not pulling enough air. I'd recommend a factory flex fan and be done. My 273 runs fine, no shroud, but with a 3 core radiator.
I'd run champion N-10Y same as a Commando engine. Low compression 340's run the same plug as the high compression 340's. Set timing at 10 degrees to start.