[Tom Goldman]

Most TH 400 trans brake applications require .060-.080" clearance.
even with that clearance there is still considerable drag in the pack.
Most builders use o-ring segments slightly thicker than the disc to stand off the steel plates from the discs to reduce drag.
4 to 6 segments per disc,cut so they just fit into the steel plate slot in the drum.
the disc and centrifugal force will keep them in place.
most 400 brakes also require an .030-045" bleed hole in the drum and heavier piston return springs.

The drag will be even worse if you are using metallic waffel type discs.