Can you count the number of wheels since 1950 that have been weighted and balanced using lead weights? There is something wrong if your wheels are not the same as everyone elses. This is pure speculation, I'm no expert, Bill would know...
The tubes in tires can move, scrunch up, the rubber can be thicker in one place, thinner in the other. The tube type tires are not manufactured to a spec that would help. A perfect solution for you is like Martin said.. Put the old ones in a safe place and get some 1 piece, true radial rims and put modern radials on the truck. Problem completely solved.
Also, if you are experiencing these problems over 45mph, you are going too fast. These trucks top speed was 50 tops (in reason) in its stock form. You are the stock guy. Don't they also put a liner inside the tire? So many factors and why they went to radials.
Sounds to me like you are having problems way beyond which radial to use. If you take radials out of the equation, I have no idea how you can make it work better. Its a farm to market vehicle in its stock form. Good luck!
