How much does temperature affect motor?

Oh Boy. Another train wreck coming.
Hate to see someone waste a bunch of time and possibly their engine.
So I'm going to put my .02 in

The reason the engine had to be jetted down is that there was more velocity under those conditions than the engine that carb was designed for. Both the primary and secondary blocks of that carb came with power valves. The primary power valve was 10.5 because the engine was a hottie and with somewhat poor part throttle combustion efficiency needed to go richer earlier.

You've got
'76 360 out of a motorhome, edelbrock performer rpm intake, and unknown brand headers,

@Wyrmrider hasnt been on for a while which is a shame, cause he'd probbaly know a fair bit about that engine.
It very likely has a fairly moderate cam, with lots of torque and power at the lower rpms. It may also have little passages under the exhaust ports. (When you put the headers on, you should have see them and plugged them. )

Assuming the fuel tank has summer fuel, the place to start solving the issue of dying when warmed up is what's going on at idle.
For that jets dont matter. For that the timing curve doesn't matter.
Base timing may matter, but more important to solving the problem is figuring out what is different after 15 minutes.
The possibility that it was running rich at idle cold, and then is too rich when warmed up, is worth pursuing.
> This could be too much pump pressure for the float position (you would see fuel dripping out the boosters)
> This could be a damaged power valve or PV gasket (either bowl).
> It could be because this carb is intended for an engine with lower vacuum at idle than your engine. However I'm leaning against that being enough of a difference in calibration to kill the engine.
> It could be gaskets have dried a bit (common with Holleys that sit depending on the gasket material) but usually there is some seepage visible externally.

Another possbility mentioned was on the fuel delivery side. If the fuel is getting too hot in the lines, this can push either excess vapor or sometimes alternating liquid fuel and vapor into the bowls. That generally results in low fuel (liquid) levels in the bowl, but can in some situations, usually after shut off and restart) cause flooding. So far the symptoms don't seem to match.

PS. I would not make any non-reversible changes to that carb because its relatively valuable especially if it has all its original pieces (other than vacuum pod cover). Changing jets, adjusting throttle position, secondary springs, pv, that wont hurt its value to collector.

Bottom line is to start with the carb. Make sure the gaskets and PV are good and the fuel level is correct and staying correct.
Something is changing between start and warmed up so keep on until you figure out what it is.