If you haven't changed the angle of the engine or the rear the angles will be correct. In order to the correct drive shaft measure from the center of the front U-joint to the center of the rear U-joint and a driveshaft company can build you a driveshaft from these dimensions. I have driveshafts built quite often. Trying to find a used driveshaft that will work isn't worth the effort. Put the slip joint in the transmission, measure from center of joint to center of joint. Take the slip joint and the rear joint to a drive shaft shop and they will build a shaft. I took trig in high school and college. But you are trying to make something complicated out of something very simple.
You are giving sound advice. However, my salvage yard driveshaft has been working fine for several thousand miles. I posted to basically report that the DS angle changed less than 1/2 degree even though the driveshaft was shortened more than 4”.
Doing what you recommended above is how I did my first conversion when I needed a shorter driveshaft.
