The hardwood block is plenty tough enough and the balsa ribs would let go before it would. I'd add another doubler to the ribs where the landing gear blocks are glued on. If you had some lite ply that would even be better as an additional doubler. Try to make it cover the length of the rib from spar to trailing edge as much as possible if using balsa. Remember the more surface area covered by the doubler on the rib to spread the stress, the better. if using lite ply then a piece 3/4" wide that would run from the spars to the trailing edge would be fine. 3/32" thickness of the doubler would be fine. Remember to also use the one provided with the kit. You want to add and the additional strength.

Cheer's