17.5's would be very unusual on the 1/2T, special order - the 2nd spring pak you have is called overload springs, and just as likely to be aftermarket, not part of the 8 leaves in the manual, which are the standard leaf pak [mine has overloads too]
the standard 1/2T's had either 15's, which gives the base GVW of 4000/4200#, or 16's for the higher 4600 - check the door plate to see what GVW your truck was sold with and you'll know what tires it had, only would have had 17.5's if it says 5000#
the 700R15's are very close to the 6.70x15's, tho being 10 ply [truck] tires are a bit harder ride than most folks want ... the 235/70x15 Fred mentions are approximations, but lower profile and wider, like all passenger metric sizes, which have the softer ride folks are used to nowadays ... P-metric sizes larger [first 2 numbers] will wind up wearing the pitman arm on left turns
Bill
