I don't know how much should be there but I would say there should be a gap. When I assembled mine I had none and tried all kinds of ways to get a little gap because I kept scratching the paint. I must have repainted those aprons five or six times. I kept changing out the front blocks until I accomplished somewhere around 1/8", I hope somebody with a properly restored or original truck chimes in with the right answer for you.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.