the transom should be beefed up a bit no matter what the motor weighs....and the weight isnt usually the real problem, but its the thrust of the prop under adverse conditions in combination with the weight.
when i say adverse conditions, I mean at WOT when you are trying to push past hull speed, or driving into wind and wave action...
motors up to about 125 lbs (10hp 4cycle) have been mounted on the transoms, using a bracket and a good strong plywood backing inside the hull, but at that weight, its usually the undersized bracket that is the major concern.
personally, I like a backing plate on both the inner and outer side of the transom to better distribute the force over a larger area (stainless or aluminum outside), but i have seen some pretty light mountings without backing plates that have stood up to the abuse....
a 60lb 5hp motor wont be too much different than a 40lb 6hp.... as, while you are increasing the weight, you are at the same time decreasing the thrust/leverage applied while under engine power...
