The superstructure of this kit is built on its own baseplate which is bolted to the footplate.
This baseplate  the cab floor  and the footplate support  were removed from the fret and the tags filed off. Noting that part  has two tabs that should not be mistaken for tags and not removed.
The baseplate is attached to the footplate with seven 14BA screws and nuts through the holes etched in each part. There are nine holes but it is suggested that either the two at the cab front or the two at the back of the engine compartment are omitted, I chose to leave off the latter. I was expecting some difficulty after my previous problems with nuts on the footplate, but what really surprised me was the degree to which the holes in both the the baseplate and footplate had to be opened up to take the bolts. I can see why they were etched this way because they are so close to the edge you are left with a very narrow bit of brass at the corner, but this instruction to open out should be made while they are still on the fret and there is something substantial to hold rather than the part assembled footplate. In fact given the size of the nuts, my club fingers and the general propensity of such parts to always find their way into the lowest and most inaccessible corners of my workshop floor I think this task would have been better performed on the flat etches. Whatever you do do not hold the assembly across the battery boxes or rear steps as you drill these holes out.
Moving forward, it is important to liberally coat the area of contact between the baseplate and footplate with solder mask or grease to prevent these two part becoming joined by stray solder during the following steps. Then after bolting the two parts together with the nuts uppermost the nuts were soldered to the baseplate. All seven went down without further problems this time.
I cannot emphasise enough the importance of using plenty of solder stop/grease and remember how the footplate was formed, by soldering two layers together in the same inside edge that is now exposed to the flux and hot iron.
Now the baseplate was carefully removed and any of the nuts protruding over the baseplate outer edges were filed clean.
The footplate was then bolted back on to the frames and the inner cab floor  was soldered in position, there are half-etched grooves under the floor to coincide with the frames.
The instructions state that the footplate support  should be fitted to the frames at this point. I've scrutinised the drawings but cannot identify the position of this part. By its description alone I think it may be placed across the frames at the centre. If I have this wrong it can probably be removed without too much pain. If it is correct it will provide some useful support here, though I'm a bit worried about it restricting options for motor and gearbox.