In 1935 the measurement standards for wheels and tires had in recent years changed. Gone were standards that based sizing on the outside diameter of the tire, known as "high pressure" tires, and in was sizing based on wheel size. The diameter and width were now measured at the base of the wheel's bead seat. These tires were called "balloon" tires. This is the reason your truck's tires have two sizes shown. The 30x5 is the "high pressure" sizing, and the 600x20 is the "balloon" sizing. The below early 1930s chart shows what equates to what.
I guess it's your truck and you can screw with it any way you want, but cutting up old 20" wheels and squeezing their centers into modern 16" tubeless rims and tires won't be a good solution IMHO. Besides looking like heck they'll rub badly on the extremes of the turning radius since any modern tire will be too wide for the truck. Places like Coker Tire, Universal Tire, M. E. Miller Tire, and Lucas Tire sell reproduction tires in old sizes to fit your old rims. These will cost you money, but won't ruin the value of the truck in the process. That'd be my suggestion. Stu McMillan
http://i286.photobucket.com/albums/ll89/truckdog62563/Wheel%20Profiles/rimtireconvertionchart.jpg 
