I have done this job twice now, or four times if you count both sided, anyway done two cars, a TR4A and a TR250. They are the same as the TR6 in the sill area. Couple things I can think of some maybe obvoius, it is easy to fit them so that they are off, too low mostly, but can be to far back or forward as well, would have to work at it to get it too high, of course you want it straight with the door gap as well, so fit the other panels and make sure all the gaps are good before you fit it.
The old ones are a bit of a bear to get out, along to top and bottom seems is mostly a lot of work, I just drilled out the spot welds, then welded the holes in the existing bodywork to the undrilled new sill suface to mimic to old spot welds pretty well.
The areas by the a and b posts take a little more finess to get out without mucking up the parts they attach to. I used a cold chisel air cutter, sawzall and dremel with a cut off wheel depending on where I was and what worked/fit/did the least damage.
I did not get the old rocker out where it is sandwiched at the top of the front inner fender, but merely cut it off a half inch out or so and welded the two, it just looked like an inordinant amount of work and cutting to get it out of there.
Both cars were done the same way, except I brazed the work of the 4A I did 20 years ago, although I don't have the car, I have seen it and the work has still held up with no rust through or popping of the brazed joints.