ok, much later I know, but here is what I did last week!
Stripped out the series seat box, took off the sides and the back angle, they were well rotten!
Took apart the 110 box but left the sides attached to the back angle.
Attached this to the series part for a perfect fit! Even the holes on the rear tub match up, hadnt changed in over 20 yrs!!
This was much less work than making the 110 box fit, I would have had to swap all the seat runners, mod the boxes and the tunnel box, handbrake hole, and add the lid clips.