Hi Don,
I would not spend so much time fiddling with the keelson: you have too much difference from one frame to the other and your notches are likely to be offset one way or another from where they should be. Concentrate on getting the all frames installed on the keel, ensuring they are mounted square to it and the jig. Then, once all your frames are in place you can check your keelson notches alignment as a whole and adjust the ones needing adjustment , then check the fit of the keelson . One step at a time... Are you temporarily securing the frames to the keel: so that if you are dry fitting them you will be able to re-install them exactly as they are during dry fitting stage?
As far as sanding the inside is concerned, personally I would not do it until your frames are in place and secured (permanently installed in the jig. If you do it one at a time at this point, you are going to introduce another opportunity for errors. All it means, is more time spend sanding when your frames are in place, but at least you can work on the entire frame assembly at the same time.
G