go here to get the spec
http://chevy.oldcarmanualproject.com...to/59tld09.htm
this is a great site if you go to the year specific index page you can pick another year that is the same style of truck and possibly glean more info if that page is missing or whatever from your particular year.
then go here to get the tire dimensions
https://www.cokertire.com/firestone-...ll-670-15.html
then go here to get a modern comparable size for height/width/rim size/etc
https://tiresize.com/comparison/
then go here for the engine rpm associated with your diff gear ratio, trans final drive ratio and tire diameter. then you can figure out the cruising speed engine rpm with a particular tire size compared to another tire size.
https://www.ringpinion.com/calculators/Calc_RPM.aspx
looks like it came with a 6.70 x 15 tire with a 4" wide rim and a 7.01 section width. the overall diameter is 28.58 so that would be something like a 235/75r15 (common size but a tiny bit taller) which is 9.3" wide or a 245/70r15 (closer to the right diameter size but maybe not as common) and it is 9.6" wide.
hope that helps