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Just a thought.
This master cylinder may be your problem.
You have a disc/disc set up.
Disc brakes require more fluid volume to operate. The fact that you used a disc/drum master cylinder may be the reason for the spongy pedal.
I don't know for sure but it's worth a look.
The reason I bought the 78 M/C is that the D60 with the dual calipers came out of a 78 and most of the posts I read people had good luck using this set up with the all disc converion. For now I will get the F350 booster and see how it works, I can always change the M/C again if it doesn't.
I tested the old booster with a vacuum guage when it was out of the truck and it will not hold vacuum, it falls off at a noticable rate. I assume that these should hold if they are working correctly?