Hmm. The pump for the L67 may be a venturi style pump designed to operate at full current only at WOT. Most of the time the pump is run with a resistor in series to keep it from ruining the pump and over taxing the fuel system. The L36 system is has different pump and no resistor. The venturi pump you installed is running full blast all the time. That's is if the pump is from an older GTP.

Check your fuel pressure at idle. The fuel pressure may be beyond the regulators operating parameters.