Stalling Help

[lowhd01]

Ok so im pulling my hair out here. My newly swapped 68 will start just fine and run great for about two minutes before it begins stumbling until it finally dies. It will restart after cycling the pump twice, only to run rough a die rather quickly. I can hear the pump running while it is trying to stall. If I give it time to cool and restart, it does the same thing running great for about 2 minutes. I have a new MAF, and 02 sensors. Computer was flashed previous to me buying it and was from a previous swap. I was thinking my return line may have been too small at 1/4" so i replaced with 5/16", did not help.

Setup:
99-F-body LS1/4l60
Inline fuel pump with Corvette filter
5/16 Fuel line supply and return
Stock behind the seat gas tank
Factory fuel pickup sending unit and utilizing the vent tube as return
No ODB

Thank you for your help!