I recently had the same/similar problem with my '56 Cadillac. All the stuff you mentioned didn't solve the conk-outs. Replacing the gas tank and fuel line fixed it right up. Something was stuck in the pickup tube or the line that would allow enough fuel to pass through but would eventually starve the engine and the engine would die.

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)