Yeah, it looks like you're out of adjustment. You may be able to weld or bolt a spreader plate to span the pocket at the rear seat mounts, add a hole lower, and drop the rear of the seat more, but I think the seat will bottom out on the floor pan before you get any appreciable movement.
Are you using pull up harnesses? I prefer pull ups, but that's the biggest downside since the adjuster isn't at the buckle like on most pull down harnesses. I wanted pull ups for the Goblin, especially for a spaceframe car since it keeps the tail inside the seat, but had the same issue as you and went with pull downs. Also, if the adjuster sits outboard of the seat, and most adjusters release by pushing "out", the lap belts may loosen themselves in an accident if the adjuster hits the seat. In other words, don't run them if the adjuster is sitting there.
A big downside of bottom mount and sliders is your seat starts to get pretty high which is bad for rollover safety, and you'll sit higher which will exacerbate your issue with reaching the pedals.