Hi potomac,
By the way, my zip is 25419, Falling Waters.....
Anyway, I have experience with Firestone, Goodyear, Universal, Lester, and a couple of others whose name I happen to forget. Its a easy choice : the Goodyear 440/450 x 21 is certainly the best, and costly too. 175 a pop is a good retail price. You can buy them at Bratton's, which is a Model A dealer over in Mount Airy, Md off i-70, or mail order them from other Model T/Model A suppliers, like Snyders in Ohio, etc.
Bratton's is also good for common parts that interchange from Model As, like light bulbs, 600 wt oils, tubes, I even use the shift knobs and pedal rubbers, etc. though you have to re-tap the threads for the shift rod, but its easy.
You can use a economy tire like the Universal brand that Coker sells, but you will spit every time you have to work with it. I know, first hand. The Goodyear is easily on top, and I actually believe, it was used by Chevy back in the day, I have some illustrations that show a Goodyear Diamond back tread pattern.
Have fun,
Jay