I dont have much info but this
315-60-15 is a 29.8" tire pictured here:
a 325-50-15 is a 27.8" inch tire, should look good on a truck
a 275-60-15 is a 27.9" tire
www.randysringandpinion.com has tire calculators and engine rpm calculators to help with tire diameter, diff ratio, and tranny options, very useful.