Some of the GMC engines of this vintage had 60 psi pumps. I agree, I would connect another guage and check.
BTW, I've never seen a 20W-40 oil. I/ve been running 20W-50 in the summer, 10W-30 in the winter. Oil pressure in my 235 runs just shy of 40 psi.
The pressure relief valve is part of the oil pump. It could be filled with gunk. First, try going to 10W-30 oil and see what happens. I don't recommend 10W-40, it has an extremely high polymer count and can break down easier than 10W-30.
