How much does temperature affect motor?

I have a '68 Dart with a stock '76 360 out of a motorhome, edelbrock performer rpm intake, and unknown brand headers, a Holley 750 4bbl vacuum secondary and a 1/4" phenolic spacer. Car came with a 26" radiator and electric fan that I swapped out for a mechanical fan (from the motorhome that donated its engine). I feel like its important to state that the engine itself has never had any kind of overheating issues... It warms up and then the needle stays glued to its position on the gauge. I tuned the engine while warm originally, it idles in park at about 950-1000 rpm and about 800 rpm in gear, and the engine has seemed its happiest at about 14-15* initial timing. The carb has been jetted down 3 levels from its original 76 jets to 73.

The problem: I've noticed that when the engine is cold, I have to drive it really gingerly until it warms up (this is expected and does not surprise me). Once warm the engine will run GREAT for about 5-15 min but then after that is when the issues come up. It runs pretty similar to to how it runs while cold where if I put my foot in it then I get a noticeable stumble, and the engine shakes a lot while idling at a red light. My current band-aid fix is to pop it in neutral at red lights to raise the rpm and then it idles great but I'm not too happy to be doing that. I have noticed that the motor in general seems really happy on colder days (70*F or less) and becomes really unhappy on warmer days especially when outdoor temps are 90*+

Possible solutions: I'm thinking it has to be an issue with under hood temperatures and the open air filter sucking in that hot air and maybe the hot intake boiling the fuel in the bowls? I want to avoid getting a hood scoop for now if possible because I like the flat hood look (I'm weird I know). My possible fixes were going to be to either remove the intake and seal off the exhaust gas heat exchange section in the middle of the intake (would this make any difference?). Or to swap to an electric fuel pump putting out 10-15 PSI and then using a regulator with a return line to turn it down to 5.3-5.5 PSI to help keep the fuel circulating and cold.

Am I completely wrong or is there anything else that I am missing?