For a while it looked like this:
And now it looks like this:
The welds will look much neater after I've ground them down and painted over them. The trickiest part was making the new sill sections. Somebody has previously welded on a new, larger box section under the old rotten sills, and the new crossmember-end repair panel was made to mate up to the genuine sill bottom, so the new parts I made had to slope upwards to bridge the gap between the two different levels. The bit I'm most proud of is replicating the rectangular depressions in the new floor section - you won't be able to see them if I put the carpet back in but I'll know they are there!
The crossmember-end on the passenger side has been similarly bodged but doesn't look so badly rotten yet so I'm hoping to get away with just patching up the hole in the floor above it and leaving it alone until next winter when I plan to undertake a more thorough restoration of all the bits of this car that have been badly bodged in the past.