You could put the keelson on with a few spring clamps, place the rib and get it's height adjusted so it touchs the removable keelson etc. I found that having the keelson attached on my Greenland style I built it was an excellent line-up device to get the ribs on the money. Once I steam bent one, I would place it in position, overlaping the gunnels and held with spring clamps until cool and stiff. Then I would fine tune it's height (including the mortice) for a perfect fit. Only the first and last one had to be made shorter to squeeze in and there I used a small block to make up the difference where it meets the keelson. This is done all the time on skin boats. I found having the keelson attached an advantage. It's also easier to establish you exact rocker as well.