Ted,

15" wheels were an option on all 1/2 ton trucks from 1947-1955. Buy a high profile black-wall tire and it will look fine.

Also, many 15" tires have a diameter similar to the 16" tire's diameter. The original 15" had a bigger diameter. This would increase the speed at a given RPM.

http://chevy.oldcarmanualproject.com/chevyresto/54159.htm

http://www.coopertire.com/html/tireselector_bySize.aspx?twtd=235&artw=75&rd=15

Tim