Gearbox removal

So I've bought a shed...

I've got a new gearbox to fit as the plug full out bringing her home, draining all the oil and then was slipping in every gear.

So I've started out by trying to remove the seats and seat box to find that the seats have been nut and bolted so they're just spinning once lose. So I'd have to take out the fuel tank to get to the nuts

How much of a pain is it to remove and refit from underneath? do have access to a pit if it would help...

you dont need to remove seat box ,it does come out from underneath removing tunnel and using an engine crane to lift and lower will help

That's good to hear. Im planning on using a crane like you say but thought it would have to go through the seat box. I got the floor and tunnel off this afternoon before work but had to shoot off before I really took in what I could get at from there. Does the handbrake get in the way?

Good case or read properly before asking

disconnect hand brake cable at the lever,
remove the crossmember
which engine ?

Disco 300tdi on defender lt77 box and disco rear axle

place a bottle jack on panhard rod and lift/support engine on front sump edge ,makes it easier to remove and refit box

Sounds like I've got the right idea for everything else I've read. Do you think it would be an advantage to do it over a pit?