Hello Martin. It seems like disc brakes should have a power booster, you don't have to push the pads very far but you have to push them pretty hard. I was running a Volare front clip under my '55 Merc pick-up; discs, without power assist, [boy what a chore to stop], then I put a small booster under the floor with the master cylinder and fixed the problem. I didn't have to lengthen the rods on either my '55 or my '36 but I think it would work. One advantage to lengthening the rod would be to get the power booster back out of the way, because it get kind of busy right under you feet. You still have to fill the Master cylinder once in a while, so keep that in mind.
Keep on tinkering.