[Jac Mac]

Spring should be 0.900" long -free length.
~0.290" dia, and ~0.600" long compressed.

Check with a small mirror/torch if possible to ensure that the other portion of the spring is not still in the hole. When new replacement spring is fitted it should be approx. 1/8" (0.125") below the surface of trans case prior to refitting the 9/16 bolt ( eg the bolt should contact the spring 2 or 3 turns before it is tight. DO NOT try to use a spring that has a longer compressed length as it will prevent gear engagement.

Three other points:
1. Try modifying your driving style- use the throttle to accelerate & the brakes to slow down, using engine braking with that big stroker combo puts a lot of reverse loads into the system and don't help in this regard.
2. IF that spring has been cut prior to trans assembly it raise's a red flag as to the quality of work & care went into its build.
3. If you know someone who works with ford top loaders ( both 4 and 3 speeds have them-3 in the 4 spd & 2 in the 3spd) try them for a spring, or IIRC you can get a ''small parts kit'' from David Kee trans, Summit etc might also sell these. BTW the flat washer should be under the bolt head as you describe, its a soft metal sealing washer.