Originally Posted by Max...
I don't think having them on the tube/floor transition is a good idea. I'd want much longer bunks to support the length of each tube.
Disagree (with the former point.) I think the best spot would be right there - that's where the floor edges are in contact with the fabric bottom, so you get solid support without stretching the floor material. At the floor/tube join, you are supporting the only rigid part of the structure (aside from the transom, which should also be supported); having the bunks further out means the tubes have be solidly inflated to hold the boat up (which would not be ideal if you have a burst tube but still need to get the boat home.)
As far as the length, most of the weight will be towards the back (usually), so that's where you need the support.
On my SIB, the bunks ran from the back, about 3/4 of the way forward, and extended several inches behind the transom. A single center bunk at about 1/3 back supported the keel, and a bow stop (basically a bunk mounted upright) located the front of the boat at the winch post.