Hi Steve, I have no idea on pressures, but I love the paint job! I'm a proud new owner of a mod IV that's been setting for 10 years. My first purchase was large smooth tundra style tires for my grass strip. If those are Goodyear "big plane" tires they are plenty tough. Maybe you could see how they behave by starting at 25psi and do a few touch and gos. Then drop 5psi and do it again until you get where you want to be? I wouldn't get too crazy low and blow a tire off the bead on landing though.
Jay