Here's a bit of info on rim width to tire size:

http://www.dunloptire.com/care/widthRatio.html

http://tireweb.tomstire.biz/products.htm?prodid=pirelli23&F2=&gid=3812&bcname=pirelli

Check with the tire maker, I think 7"-9" is the reccomended rim width.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi