Sorry, I've just seen this.
You're right that there was some jiggery-pokery in getting the spacing right initially. My universal runners came from DT so chances are you aill soon have the same as me. The universal kit consists of 2 L- shaped sections which I bolted securely to the floor (had to drill a couple of holes to suit the spacing front - rear). There are then 4 risers which bolt to the seat and to the side of the L section. The fronts I believe do not sit quite vertically in my installation as the seat is wider at the front than the back. It isn't far off vertical from memory as when you tighten everything up it pulls straight.
I think perhaps one of the rear risers was mounted inside an L section to allow for the narrower rear, though I may be wrong.
In terms of harness mounts I drilled the tunnel and used a reinforcing plate with nut welded on it and bolted the original seatbelt clip to the tuunnel using that and an eyelet which the harness clips onto. I used the same eyelet bolts at the bottom of the B pillars and also where the shoulder straps reach the rear seatbelt mountings, though the last I would not use without a harness bar to modify the angle.