That's correct, and it won't fix worn bearings either. Assuming the pressure relief valve isn't stuck, a heavier duty spring will just raise the PSI point where the oil is dumped back into the pan. A stiffer spring might bring the cold or high RPM pressure from 65psi to 80psi but it won't effect the hot idle pressure as the valve should be closed at that point, assuming the spring is good. If the spring is bad or broken, you should see a drop in the high RPM reading as well. If the bearings are badly worn, raising the high RPM pressure limit might actually cause the bearing to spin easier. If keeping the original block is important to you, I'd replace the bearings before you have a failure. Slant 6's have a bad habit of slinging a rod through the block when they fail. I've seen many lose #6 rod bearing where it slung the rod through the block so violently that it hit the starter and broke it off too, ruining the transmission in the process.