If you're going to metalflake it you need wider gaps but otherwise, I think i's more important that they all be even top to bottom and the same rather than a particular size. Cadillac usta have the thickness of a half dollar I think. Thickness of a wood paint stir stick is safer. Won't chip the paint like the half dollar will.
3/16 or 1/4, either would be fine, or better yet why not go with this......as long as they are all even!!! Better yet, what is acceptable to you? It's your car, you have to pay for it AND you have to drive it!!!!!!!
Personally, I follow the time proven methods of the finest German craftsmen in establishing my panel gaps:
Sweet... More tie psent ragging a man over his word choice than people standing up with serious technical advice. I agree with the overuse of rat rod and people feeling wrongfully grouped. I feel the term Rat Rod is the most misguided term on earth. Do your homework rat rod builders....
Got one side done.. One side to go.. and its been a bear.. the doors are heavy.. the body is week.. Had no floor to start with.. that didnt help matters much.. Im at a quarter inch gap on the P***enger side..