Well, if it is a 1998 model (on a 1997 chassis), you probably have the updated "Vortec" version of the venerable 454. That was the final, and best iteration of that engine. It has Sequential Fuel Injection, new piston design and improved pushrods. All that being said, 58 psig seems a little low for the SFI engines at idle.
When was the last time your distributor cap, rotor and wires were replaced? The 454 really liked to cook the rearmost wires, and the coil gets weakened after several heatsoak cycles. Those are other items to check. Since it is OBD-II, did the ECM store any codes (active/pending)?
