While doing other work on my 1989 E350 (Telstar RV) with the 7.5 460 gas engine, I just noticed a crack on the head for the #1 cylinder (see pics) drivers side rear most cylinder. I know it isn't good news, but how bad is it to run with a cracked head?

It may have been cracked for years. I've noticed no symptoms and the engine seems to run smoothly. I get 7 to 8 mpg but am told that is typical. If I have to pull the engine is there anything I can do to improve the mileage while I am at it?