This knotty problem again! Always rotts, is thinner than other chassis members and is a pig to get out.
Remove the sill panel and door fill panel (the little triangular one whose rivets always fall out, you only need to do this on one side, but it helps with visibility if you do both
Dont bother trying to undo the various nuts and bolts simply cut them all off.
Prepare your cross member; if you have not got one that has separate brackets drill through them and grind off the welds to separate.
Remove the small rectangular brackets the bolt to the section that supports the now removed sill
Cut it exactly in half through th middle and prep the ends for the final weld.
Now for the fiddly bit, insert the cross-member if you are only working from one side get underneath and put in the section furthest from the removed sill and fill panel, it can be just put in place by first putting it in longways on between exhaust and propshaft. The screws securing the floor were fouling the top so I had to trim these down. The were badly coroded so arent going anywhere.
You may need to make some re-infrocing plates out some aluminium as the body work behind the brackets often corodes away into white solihull powder.
All bolted up loosely so that weld can be aligned.
Not the best weld I've done but its strong enough and all covered in waxoyl - so no going back!
All rivetted back together now, that'll keep my MOT man happy now.
If I was doing this job again I'd consider getting a galvanised one with removable brackets and try getting in all in one first. You could dispense with the need for a weld if you made a section that bolt the two halves back together.
Hope this helps and is considered fit for the technical archive.