Our Niagara had holes 4 inches apart and I drilled new holes half way between the existing ones on the track. The holes only have to go as deep as the stopper pin and using a cordless drill it was very easy to do this without going too far. I did not use a stopper.
Our current boat has an adjustable car system with lines, etc.. .and is certainly a much nicer system - especially the leverage to move the cars forward under load. However it would indeed be expensive to convert as you would need new cars, blocks and cleats on each side.
I think your plan is feasible however inserting three holes between each of the existing holes could be a concern. Why not just try one for this weekend and if it seems fine you could always add the other two?