More than likely the steel lines that you currently have are not large enough (specifically the return line) to flow the necessary amount of fuel required by the setup.
You will need at minimum a 3/8" supply and return line. Most of the return lines from the factory on these cars are actually more of a vapor return that is not designed to pass much actual fluid. Most of the time they are 5/16" and may crimp down smaller in certain places. It's not adequate for returning lots of fuel back to the pump. The net result is that the return line starts to build pressure in the line, which then increases fuel rail pressure as the pressure regulator is unable to cope, then you get an oscillating rich/lean condition that causes surging and stalling of the engine.
For long term use without the need to replace lines, going to a properly sized stainless steel hard line is preferred. That said, if the car is not used often, is not driven or stored in lots of inclement weather etc. you can certainly do EFI rated soft line.
This kit is cost effective, contains quality parts and is everything you need for the fuel system.
https://www.tanksinc.com/index.cfm/p...rod/prd548.htm
I have been running that kit on my wife's chevelle now for several years and my Firebird has a piece meal kit of the same type that has been in-service without issue since 2015. Just know that a soft line system will not last as long ultimately as a proper stainless steel hard line system.