Best timing for a 454?

[Old65C10]

I am getting ready for a trip in my 84 K30 Service truck. It is a 454 with a THM400 and 4WD.
The factory sticker says the timing should be 4 BTDC, but right now it is at 14 BTDC. It seems to run OK and I do not hear any pinging under load. But, I was wanting to set it for the best MPG.
This is a former Fire Truck and it only has 19K miles on it and has excellent service records. It has recent tune up and new wires, air filter, etc.
Should I leave the timing where it is, or change it?
Current mileage is 7 MPG, so anything would be an improvement.