The suspension and steering geometry of any 1950's car was designed around the general characteristics of bias-ply type tires. Rims used on 1950's cars were generally set up for bias-ply rather than radials, in fact some types of wheel rims do not seal properly for tubeless tires; I had a 1951 Kaiser like that (or perhaps, with age, they lost their sealing abillity).
It IS a matter of personal preference on what tires to use, but for judging purposes, the 5.90 x 15 tires (size used on Allstates) would be correct. What Allstate sales materials I have do not show a 6.40 x 15 option.