Series 3 seat box interchangeability

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?

On my Series 3, the previous owner put a new seatbox in. I believe our hosts sell them, too. It's a bunch of steel or aluminum panels with simple bends and they're riveted together. It ships flat packed, you assemble.

My seat box was installed with unequal spacing. I needed to install original seat belt brackets for retractable 3 point. Drivers side had plenty of room, passenger side not enough. I was worried none of the holes would line up if I nudged it, and I was right. That's because they're pretty much drill-to-fit. You can drill new ones, and use M6-15mm-1.0 bolts for the outside and back(against rear bulkhead) and flat-topped, or wide-rounded floor screws with those speed nuts for the floor panels.

Honestly, it's embarrassingly simple...just flat sheet metal bolted together. None of the holes are in the same spot, and I would think most suppliers ship un-drilled so you can decide where you want the holes. I wouldn't worry about matching up holes, you are not likely to do it. If it bothers you, you can fill them (spot weld and grind back) or get rubber plugs. It's going to probably be under floor mats anyway.

Your problem of the gearbox cowl is more of a thing. I would get the Defender seatbox if you know it's the same size. You might be able to get the panels separately, rather than the kit? I've definitely seen our hosts sell the sides of the seatbox separate. Maybe you can assemble a franken-seatbox with a Series seatbox and the front from a Defender? Unless I'm missing something.

Also, a metal supplier might be able to fabricate a custom seatbox for you pretty cheap. It's like 6 x 90 degree bends.

Here you go:

https://www.roversnorth.com/parts/rn...apter_for_r380

Looks like it comes in a kit, too:
https://www.roversnorth.com/parts/rn...380_kit&type=3

Here's the seatbox, too(after-market. Its unreal what genuine seat boxes go for >$2500!!! This one is about $350):
https://rimmerbros.com/Item--i-LL1962

YRM Solutions does some nice sheetmetal work for LR's:
https://yrmit.co.uk/product-category...-3-swb/page/2/

Our hosts sell YRM stuff, too. For the life of me, I can't figure out why RoversNorth.com doesn't show some products I KNOW they carry. The printed catalog has items the website won't show you. Give them a call, I guess.

Seriously, search "Seat Box" and you get no hits.
But search PLC069 and look what shows up:
https://www.roversnorth.com/parts/pl...series_iia_iii

What?