Who;s kit do you have?
I know how Classic Roadsters handled this.
Classic Roadsters left the master cylinder in it’s stock location and provided a steel pipe to run between the brake pedal and the master cylinder. Once you relocate the pedals, you cut the pipe to he necessary length, tapped both ends and screwed the VW parts into the pipe. As they thread in, with a locking bolt, you have about a half inch of adjustment range to take any slack out of the set-up.
You used the phrase mushy. A soft pedal usually mean sair in the brake lines. Adjusting the brake rod will not help that. You need to bleed the brakes.
Hope this helps
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)