I just went back and read your original post and at one point you mention how you can easily move the
oil pressure relief valve with a screwdriver.
I am assuming you are speaking of the relief valve in the back of the block and not the one on the
oil pump, I am assuming that speedpro has a pressure relief outlet .
On my sideoiler I eliminated the pressure relief valve/spring in the block and shimmed the
oil pump pressure relief valve spring to achieve a higher and sustainable oil pressure, I'm using a Melling HV57 oil pump.
This leaves me an idle pressure of about 70 -75psi and a "heavy +" oil pressure of 100psi @ higher revs, 2500 up through 6200 rpms.
I don't believe that the pressure relief spring that you moved with a screwdriver is supposed to be that "soft", my understanding is that you should be realising a lot more resistance out of that relief spring than you describe, it should be worth a look, possibly a spring pressure test before you put it all back together...good luck.