I suppose the EGR could get plugged in a way that tends to direct exhaust gas one way but I wouldn't suspect that first. Are you using the stock TB with the diffuser plate on the bottom? Check to see if the plate highly carboned up.

Most of our GM buses are mainly around town, stop and go. My testing has shown improvements in mileage if EGR is disabled during these conditions. Unfortunately I can't leave them set that way.

My suburban is showing about 10-12% rich in closed loop. Initially I suspected across the board timing changes were needed for the 302 but those are not bringing O2 into line. I'm going to return to stock timing and see if the EGR is sticking open next.