68 GTS won't stay running

check ur wiring under dash and fuse box , If u have good fuel pressure using a gauge at least 5 psi that's a problem is ruled out .. if no spark after the stall n cut off then its electrical issue .. Where talking about a 30 year old wire harness correct ? u converted to ECU ignition u need more amp's to power it 60 amp alternator a constant output regulator the blue box .. check ur fuse box turn up side down , may need a new one for that set up.