[TRS]

You changed the fuel pump, correct? As you said, you are running return less. Nevertheless, exhaust heat can bring the fuel to pretty high temps. (in case of some GM vehicles the corresponding expansion and contraction effects of the fuel are used for leak testing of the fuel system, what excludes the necessity of a active vacuum pump as found on the 86)

Another aspect is, that unnecessary high powered fuel pumps can also add a bunch of heat.

In your case, checking for vacuum in tank is pointless on a returnless system. The system is meant to be sealed and under more or less strong vacuum (based on current operation condition) by the EVAP purge valve due to emission regulations. The vented tank as found on older cars or lawn mowers is part of automotive history meanwhile. There is even a specific test unit (EVAP canister) containing a vacuum pump which can be found on US models. CA evaporative emission regulations are very strickt and a automatic test System, checking if the Tank is still 100% sealed every few days, is mandatory there. A Tank which is not able to hold vacuum would cause DTCs on those vehicles. In other regions the system works basically in the same way, but the automatic test isnt mandatory. This means the system is still meant to be sealed, but the test unit isnt installed.

The regulator is working against the pressure in the main fuel line and doesnt care about pressure difference between ambient and in tank.

But in your case, I would check the intank pressure regulation valve first. All connections on it to begin with. (if something is leaking there, you will never find a leak looking from the outside, since leakage volume is flowing directly back into the tank) If you dont find anything there, I would replace the valve.

All this ofcourse after you made sure that realy low fuel pressure is causing your issue by measurement. If you have any wierd table changing the fueling due to high ECTs or you actualy have a overheatung issue, the fuel pressure is maybe not even your problem.

If its realy a fuel pressure issue, I would assume the regulator valve or a connection to it is reacting to the high temp.

In another application (return style system) I reached a point where the (aeromotive) fuel pump stoped working due to overtemp. I installed a cooler in the return line later.

Concerning fuel pump: I dont know the specs of your swap, but if your LS isnt FI or heavily modified, you should be fine with the OEM pump. I didnt had any starvation issues with mine. A well sealed trap door and a good working passive side pump is much more important for non FI engines (also a stock or mildly modified LS) to prevent starvation than a high flow or high pressure fuel pump. As already said, this will in the end just heat up the fuel, since the pump is cooled with fuel but generates more heat as a high power unit and also a high overflow rate at the regulator will generate additional heat. (the 86 uses a a simple on/off fuel pump signal, it is not a digital, PWM controlled pump, meaning the pump is always running at 100%)

High power/pressure pumps are something meant for FI engines with intake manifold injection (as also used on the 86 in addition to the DI), since the pump needs to work against positive MAP pressures in those applications.

BTW, is the OEM heat shield under the tank still installed or did you remove it for and reason?