Yes, the forces have to end up somewhere, that is to the forks\frame, which can get pushed past the limit. A friend got disc brakes conversion done to his 57' 2000kg american car, brakes were immensely improved, yes, to the point of on first heavy brake test the bolts securing the front calipers where cut right off ! strangely enough no accident or serious damage. New improved bolts fitted, but I suggested that next time with similar brake power applied it could be the frame\front suspension\steering parts and more beeing cut off, recommending him to be -very- carefull.
These vintage things are simply not constructed to withstand the immense force a very good brake system will inflict upon parts, needs including some strengthening on many areas of the frame\suspension etc., Mike, you seem to have discovered all this, good luck with the repairs.