Well yes it will; your exhaust temp at idle will be near 400*F. It will only get hotter as the engine starts to work. Typically at WOT, temps can climb to hotter that 800*F.
That will NOT be the temp in the passage, altho it is common to find that chamber plugged with burned carbon, that came from the engine oil.
If yur looking for mpg, you gotta plug at the very least, MOST, of that port so it cannot cook your carb, and mess up your AFR. For your application as described, I would block it completely. AND, the carb should be getting fresh cold air from somewhere NOT under the hood. As long as it is sucking underhood air being cooked by the rad, the hot engine and especially the hot headers, Your AFR will be impossible to regulate properly. You do not need a scoop or a hole in the hood, just some ductwork past the radiator core support.
For an all-out mpg effort; your cruise-timing will need to be optimized. Yur gonna need a ton of it. Depending on your cruise-rpm, 50 degrees might not yet be enough. I have run my low-compression 318, into the 60s, with pretty good results.
With headers, your 318 may over-scavenge at too low a cruise rpm. You may find that it gets better economy at 2000 than at 1600, which, theoretically, should not happen.
To find the best cruise rpm, tee a vacuum gauge into the intake, then, in P/N slowly rev the engine up. As the rpm rises, the vacuum should also rise as the engine gets more efficient. Around 2400 with a stock cam, the vacuum might start to fall. Find the plateau of highest vacuum, and try to cruise as low as possible on it.
If you have 2.76 gears and 26" tires, this might be 65=2400. But if your cruise timing is only 24*, yur never gonna get decent mileage out of it. Not even at 34 degrees, and not even at 44* degrees.
To find your target cruise timing; in N/P, rev it up to your specific cruise rpm and hold it there while simultaneously advancing the timing; just pull on the can and close the throttle until more timing no longer produces more rpm. Now read your timing, and subtract 3*; that will get you a target to try to aim for. Reduce your rpm and reset your timing back to where it was before the test.
In all probability, at 2400 rpm this target will be over 50 degrees. I kid you not.
The engine is telling you what it wants. Now giving it to her will be the challenge.
