I’m building / restoring a modified Series 3 that includes a much larger transmission. I have to replace the existing seat box because every panel is corroded BUT a standard Series 3 seat box doesn’t fit due to the transmission tunnel clearance. I need clearance that the R380 provides. Do I purchase a new series 3 seat box and then cut / replace the tunnel from an R380 or does it make more sense to buy a Defender seat box that already has the R380 tunnel?

I know dimensions of the series seat box and defender box are the exact same. If question is are the bolt locations to bolt it into the door sills and other locations (floor panels) the same on a series and defender?

TL;DR - are Series 3 and Defender seat boxes interchangeable outside of the transmission tunnel and the seat rails?