I bought the same pedal for my 65 impala. Ran into the same problem and bought the spacer they sell. Pedal didn't come with much directions, but you can adjust them by loosening the screw were they pivot, but You probably already knew or figured that out
I had the same problem when looking for a shifter. I found an out of the box hurst pro matic 2 for 100 bucks. it is a ratchet shifter in 1 2 3 and 4th so it will work with a reverse valve body but I had to add the extra gear. it already had an extra tooth to go one way. I had to weld on it a little bit and finish it with a file to add one more tooth to go the other direction but it works great. check it out. you will see what I am talking about.
you are right. it seams like every part needs to be modified. whenever possible i make parts myself. that way i cant complain if i have to redo it three or four times
