@lgardner - Louis my friend, that’s a good question!
I don’t think it’s necessary to thin either the side walls or the fuselage area. The problem comes from the the structure itself, since there are 2 raised lines in the fuselage wall that are meant to guide the positioning of the cockpit tub. However the latter has 3 sections, but I believe the most rearward is definitely not to be positioned within those lines.
Fitting the parts within those lines seemed easy when dry fitting but not once I glued the fuselage. In hindsight I would suggest glue the fuselage halves first and then insert the whole tub. Any troubles found in that step can still be rectified with ease, since fuselage walls are bare, no details there to be lost with sanding or grinding.

As for the seams, well I’ve had far worst 😉 more progress soon I hope

I always have problems with kits that don't have positive locating points for the cockpit tub. I end up using TLAR (that looks about right), which is a 50-50 proposition at best. The results you got certainly look good, Pedro (@holzhamer).