Hy John D, I'll try and answer your questions.
1. yes there are a multitude of different speedometer driven gears based on rear axle ratio and tire size. I think the main reason your speedometer is reading incorrectly is the change in tire diameter that you have made.
2. yes there are synchronizers in your transmission, it sounds like it may be time for a transmission overhaul. There are many choices for newer fully synchronized transmissions some even have overdrive so more speed at the same RPM.
3. I would say 3200 RPM would be where I would hold it if you were going to be at that RPM for a while. When your truck was new common "highway" speeds were 50 mph (80 kph) today traffic travels much faster and as I said you lost some speed at RPM by going to smaller diameter tires.
4.Tire pressures are quite a different matter today, they depend on tire construction, load and conditions. If the tire has a recommended pressure branded/cast into it I would use their recommendation or close to it. If it were my truck with a 65psi recommendation, I would run them at 50psi. Hope that helps.
