If I'm right in thinking you treat your seat effectively as a "fixed position" seat, just mounted as far forward as you can slide it, then you can probably get pretty close to what you want, depending on how, ahem, ample the Christmas tum spread has been!
BUT, it's not necessarily going to be the easiest thing to set up and make work - a lot will depend on your actual "shape" rather than outright size.
Also, if set up to do what you want, the waist straps will be totally unusable by anyone else should the seat ever be slid back, OR even if someone really slightly built were to sit in the driver's seat in it's current position.
The Pays Kool harnesses are a beautifully made piece of kit, and I always highly recommend them, but due to the pull up waist belts, and the large size of the adjusters there are some combinations of driver size and seat type that just don't work.
I had to admit defeat with these myself. I too had longer lower straps to a custom length from Luke, whose service throughout was superb. As an example of what I meant by driver "shape", in addition to just being a regular fat b******d, I have the proportions of a barn door! (probably make a good sail!!), and am wide in the hip, between that and the bulk of the lap belt adjusters, I can't "squeeze past them" if they're trapped between me and the sides of the car, so I needed lower straps long enough to hang over the tunnel and the side of the car. Because this moves the adjusters up and closer to the central buckle, it seriously reduces the amount of adjustment possible. I could get the belts done up tightly enough, but someone much thinner than me would simply run out of adjustment, way before they got the lap belt properly tightened.
Unfortunately, the combination of my and the large adjusters also caused damage to interior trim on the car, as i tried to squeeze in past the, before realising exactly what was causing the problem.
Like Dave, I fitted Schroth harnesses instead, not cheap, and no where near the bargain price of Luke's. but the pull down waist belts work superbly!0