It can be cut off with a Dremel if you can't unwind it. Then and now kits use a lighter spring, they also have a different method for holding spring and top seal on. It allows the spring to be removed and replaced easily. When I have rebuild various original hemi, and early model pumps including the 6903/4862 I have documented the part and casting numbers of the various assemblies. Also measured spring length and force. I have not had a mancini or holley version of the pump, so I am not sure what they could have done to get the pressure doubled from 7-8 to over 15psi. It would take a very hefty spring, or they have internally modified the diaphragm and housing area to significantly reduce available surface area of the assembly to force pressure up that high when flow stops. Force = Pressure x Area and the spring is the Force. When flow stop the diaphragm just hang in the pulled up position as the lever floats, until sufficient pressure has bleed off through the check valve or check valve bleeds, or the float opens the needle again.

The only other issue could be the damper diagram. It is the one between the lower and middle housing that separates the input and output chambers. It act like a pulse damper. It is thin and soft, and when the spring pushes down on the fuel in the chamber it also depress that gasket. Increasing chamber volume and keeping pressure low (some force of spring used to depress the pulse diaphragm). When the lever pulls the pump diaphragm back up the pulse diaphragm returns to original position and continues to push fuel out of the pump while the main diaphragm is starting to pull fuel in to the pump. That dampens the pressure pusle.

In an old pump when that gets hard, it no longer deflects and the pump pressure actually rises.

Old army trick for jeeps was to double up that damper gasket if the pump pressure was too low.