You can probably go to a 7" wheel without any problems. I have seen 8" wheels on them. The key is the back space which should not exceed 3 1/2" or the tire will hit the drag link. Another issue to remember is that a larger/wider tire will make the truck much harder to steer for parking. I use 225/75/15 on 6" wheels on my 56. I also have power steering.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine