Hi,
From a judging perspective, Goodyear tires--6.70 x 15 for a 1951 Kaiser--were the assembly line tire K-F used, with (it seems) B.F. Goodrich and Firestone as secondary sources if Goodyear had supply problems. Any other brand or size would be considered incorrect for judging and points should be taken off. Same goes for radials vs. the older bias-ply types. That said, there is no question that radials will give you better handling on bad roads or in bad weather, but do you want to take your Special (with its vacuum wipers) out in a heavy rain or snowstorm? I've had 6.70 x 15 bias ply type--with tubes--out in various weather over the years and didn't seem much worse than with radials in the everyday driver. This of course is only one person's experiences...how you drive, where you drive, etc will affect the outcome.
It should also be noted for those interested that the full stainless wheel discs standard on a 1951 Deluxe are listed in the K-F parts book as "optional" on a Special and a 1951 issued Confidential Bulletin shows a set of the big covers as a factory installed option on Kaiser Specials delivered retail at Willow Cottage (see Confidential Bulletin #71, 10/31/51). I had them, with goodyear whitewalls on the Business Coupe when I had it and it really looked sharp (the way a good salesman would set it up to impress customers).