Replace the ignition wires.
What you refer to as George overdrive is known in the trucking industry as Georgia overdrive. It is going downhill with your truck out of gear. This is gone down grades to allow the truck to accelerate beyond the normal top speed of the engine in to gear without damaging the engine.
This engine does not have a timing belt, it is gear-driven. Possibly, you have broken gear teeth, causing the timing to be out of order.
The engine is overloaded in case it of generator set or any other load and in case of a car the vehicle is not engage in lower gear as per load and engine rpm.
What is the gear ratio for a 1985 CHEVROLET K10 pickup
As I recall it is 12,800#.
Because you have the hand break on Clutch slipping
If it does it in all of the gears then you may need a new clutch. If it is just doing it in 5th gear then it is a transmission problem.
You will feel a jerk from the engine / transmission when putting in gear. This can inturn damage the CV shafts.
Accelerate slowly with the vehicle in a higher gear
Shift to a lower gear.
Overdrive means the wheels are turning faster than the engine so gas is saved, (5th gear would be overdrive) however if you want to accelerate quickly you will have to shift into a lower gear which would make the engine turn faster than the wheels.
4.3 L engine, aprox 16 to 19 mpg depending on diff gear ratio. 2.2 L engine, aprox 26 to 29 depending on diff gear ratio and standard or automatic transmission.