Its looking good buddy! Your sills seem to be in great shape you lucky sod!
Top tip to get all the glass media and grinder dust out of hard to reach places, get some garden hose pipe and tape it to the end of a hoover nozzle, means you can poke the hose right along the inside of cavities and hoover up all the rubbish that will cause water to nucleate.
Its a really good photo of the mid sill bracket in position, has made me think more about how to reattach mine properly.
Have you tried the pulse welding thing I was on about? I will try and do a video for you today. also on the outer skin I went for putting the new skin under (inside) the old skin and welding along the edge, once its flattened back the shape can be reformed with body filler before paint.
EDIT: It took me a while to find this one, I was looking in the build threads!!