Jeff, I have both those photos you refer to, I got them from the Boeing Archives (plus license). Looking at the B-29 with the two Grand Slams under a magnifying glass, the bombs are also held in place by two chains. There appears to be two large I-Beams attached to the underside of the wing, one either side of each bomb, the chains appear to be attached to these I-Beams. These I-Beams run in the same direction as the line of flight.
The photo of a B-29 with a single bomb hanging below the fuselage is actually a T-12 (44,000lb), not a Grand Slam (22,000lb). It is held in place by two chains too. Notice that there's an additional fairing attached to the wing & fuselage which covers some local strengthening of the spars.
Bottom pic here is a good comparison of a T-12, Grand Slam and Tallboy. I made the T-12 from dimensions I found on a website called 'Very Heavy Conventional Ordnance' which has since gone AWOL (at least I can't find it anymore after my saved link closed)
Originally, the T-12 was a direct copy of the Grand Slam, increased in size by "lofting", but it was too long to fit in the B-29's bomb bays. So the tail cone was shortened so it would fit so giving it a slightly different profile.