Understand what you mean with wanting the extra headroom just in case, hopefully you can get the real problem fixed too though. Even with the mods, and special block and such- I still don't see it creating that much extra heat in the coolant. The fuel used for just highway cruising should be roughly the same as stock- and I don't imagine your engine is significantly less efficient than stock, in that drastically more heat is being removed from the fuel/combustion charge- and less is being put towards creating power. I would imagine your setup would at least be in the ballpark for fuel heat lost to coolant as a normal LS.

Anyways- trying to think of some troubleshooting that might help- maybe try temporarily removing the IC radiator if that is blocking the main radiator at all, and see if that has a drastic affect. Maybe you could try installing temporary thermocouples at the inlet and oulet of the radiator to see what kind of temperature drop you're getting across the radiator? Maybe try taking the front bumper off completely for a couple days if you think airflow is an issue? I dunno what else there is to try- maybe if what 03 said is true- what about sticking a stock water pump back on to give that a try?

Just throwing some ideas out there to try.

 

what i learned from gm motors there tempermental with there coolent the siverado ihave has the ls engin in it. when i did the coolent flush in it i had to put more coolent then water mix in it. what i mean is it was about 60/40 mix majority of coolent in it than water after i put in more coolent in it and toped i off ith water i notice itrunning a little cooler i the heat and humidity in florida. but i do hope you figure out what the issue yu have can be fixed. i am sorry to hear you ar having a issue with heat in the engine compartment