This. I spent an absolute age on my middle sill, the spot welds on the upper track were INVISIBLE and there was no way that thing was coming out cleanly. All mangled it were and I had to hammer it back into shape and had a few goes getting it to actually work with the sliding door. I would recommend, if you need to replace the whole middle sill, (and people probably won't agree with this) getting a spare rotten sill from a breaker or similar and cutting the metal away from around the upper track, rather than pulling the track out. The tracks are usually fine and won't be re-sold anyway. Then you can measure up on the inside of the floor edge panel, drill a couple of pilot holes to line it all up, and do a nice clean job. Oh-one more thing, the schofield middle sills are great, you need to take a couple of mm off where they sit into the inner though, I found mine was slightly too tall there. Flat plate on the bulkhead- any nasties behind that would be in the wheel well, can't hide 'em. Don't worry about it as long as it's properly welded and strong.
