I put the Jackson happy feet in my boat (using directions) and really like them. Not sure about the Jitsu situation but you want to be knees bent in a comfortable position where you can put pressure on your feet and be able to press your heels down or up to position the bow for different moves.
I have mine with the seat forward and there was very little room to get the foam in. You just have to keep taking blocks off until you can force it in and then just bang it into place. It feels pretty solid now and is pretty comfy all things considered.
Cut the main foot block down so your heels can just touch and your feet splay out with your foot knuckles at the foot bump in the boat. I ended up cutting down the foot block alot. I have a 31 in inseam in the 5.9 with my seat dead center. I then have two of the insert sections attached in front of the cut down block. The balls of my feet push off of these platforms and my arch and heel wrap around the cut down block. Takes a couple hours to dial but once you are done it blows any happy crap away. It is solid and does not move around. Also give a good platform to push off of.
Could you post or send a pic of your footblock, MREKid? I"m currently wrangling with a Wavesport footblock and about to pull the trigger on some happy feet... but if I had a template or a sense for how to start cutting, I may give this one a shot.