Modern tires not seating on older narrower rims doesn't sound right to me. I have LT225/85 R16 tires mounted on the original 4 1/2" rims of The Phantom without difficulty. There may be some exceptions but I've never encountered the problem in my life and all I've ever owned are antique cars.
Your '46 rims will take modern radials without any problems but it's academic since they won't work with your brake setup.
I'm not sure the "tire folks" today have half a clue what they're doing with anything made in the 20th century.
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
| 
