Photos are good. I see what you mean. On my car it isn't quite that tight but the sides and the backrest do touch. I would just either relieve the wood a tiny bit or massage the tin part to miss the edge of the backrest. I can't tell if the backrest is a remake but if it is, it's possible the new wood and the seat aren't exactly the right dimensions. That part of the tin gets covered so any rubbing would not be severe but you need room for the upholstry to fit in there too as it will be on both sides adding to the width of the backrest. It looks very close to what it should though. I think I see punch outs for the tabs that hold the backrest to the vertical wood piece.
Looks like you are doing a 1st class job. I haven't started on my 40 convert yet, am finishing up my 40 del woodie.