Starving for fuel? Please help!

[wynschevytruck]

I have a 1968 Chevy c-10 pick up with a stock Chevy 350 with a carter quadra jet carburetor. I recently had upgraded the exhaust with a true 2 1/2" dual exhaust with a crossover from Flow master. The problem that I am having is when the truck is idling for about five minutes it shuts off with no warning. I have adjusted the idle fuel screws to four full turns and still have the problem. Do I need to upsize the jets on the carb or maybe replace the carb with a bigger one.