Wiki says that you are at about 4300 ft elevation.
I didn't see a description of what exact "heat issue" you are actually speaking of, or how you are measuring your "heat issues", but
I did see "no shroud".
The lackluster power and retarded idle/low-speed timing go hand in hand. At 4300 ft, with a 273, you are gonna need all the timing you can get without hitting detonation. Also at 4300ft, with a stock engine/TC, and below ~3000 rpm, if the tires don't spin; you cannot expect much power improvement from the 4bbl. The factory 2 bbl was big enough to that point. You will get much much better result with a hi-stall TC.
While the 4bbl does make a bit of power, it follows the cam. meaning at whatever rpm the cam powerpeaks at, the 4bbl can add to it; it is not guaranteed to, if say the exhaust won't allow it. But it also means that the power does not increase like a switch, it only starts making power gradually as the cam wakes up, and only proportionally. Say your cam wakes up at 2400 and peaks at 3900. And say your 2bbl was good to 2400. Then your 4bbl will not start to make power until the engine would have run out of air with the 2bbl; say 2400. So let say's your 4bbl was good for 15 hp at 3900, and zero at 2400. That is a range of 1500rpm, and an increase of 15hp. If the power increased linearly, and it does not, but say it did; then, you could expect 1500/15hp=1hp per 100rpm. But notice that, below 2400 rpm... in this example,,,, horsepower is not increased at all.
On another note;
Rich running cools the A/F charge as the extra fuel sucks up heat during evaporation. That extra fuel goes right into the exhaust. If it finds air in there, it will catch fire and make the logs run hot. Of course the exhaust ports are part of the log-manifold system, so they too run hot, and that heat gets into the water jacket;
and like somebody said, it's a chain of events.
But the idle HAS to be slightly rich, or you will have to take other steps to keep it running, and to prevent driveability issues.
At idle, your engine will like a TON of advance, perhaps into the hi-20s or more. You can tell how much, by simply advancing the timing until the rpm stops rising, then reading the balancer.
But you cannot run it there (hi 20s) except for idling. And the reason is that at the other end at WOT/ full load wide open throttle, your engine might not want any more than 36*@3600 rpm, else it could hammer itself to death. I mean IDK your engine only you do. So then, if you set your idle timing to say 25*, you would have to build a timing curve to get just 11 more degrees by 3600. If you did that I'll bet the engine will be hammering pre-ignition holes into the tops of every piston, on every stroke....... because at say halfway to 3600 with a linear curve, she would be at ~31* by 2100, and under full power, at 4300 ft, I would not expect your engine to last very long, due to massive detonation..
However, at Part Throttle, 31* at 2100rpm will be pretty snappy after you add the Vcan in. But this too, at 4300, might be a tad much.
At 4300 ft, the MJs in that carb needs to be jetted at least two sizes smaller than at sealevel. Couple that with at least 20psi loss in cylinder pressure, and so,
the take home is this; you are gonna have to dial the timing in pretty close, for the lil 273 to come even close to what might be considered peppy.
If it was mine, I would be exploring the timing, looking for the WOT detonation lines. In the car and by myself, this could take a couple of weeks. You will need a dial-back timing light, a tachometer and lots of paper.To make this easier and much faster, I installed a dash-mounted, dial-back, timing delay box, with a range of 15*.
After the Idle Timing is close, as determined by the lack of off-idle tip-in, which is set up by the transfer slot to primary throttle opening;
after that, I would map the detonation line, and then build a distributor curve to stay under that; then I would start adding huge amounts of Vcan timing to find the PartThrottle detonation limit.
When I say huge, I mean 22/24* is about all you can modify a Mopar Vcan to get
Now to come full-circle; To get the Idle timing into the hi 20s/low 30s , as I started this post with, You can use any mix of mechanical timing plus vacuum can timing that you can imagine. Say 10mechanical and 22 Vcan, or 14 mechanical plus 18 vcan... and so on.....
but above all things, you must stay out of detonation.
