Re: reinforce transom
You'd be overpowered, but that's not really the issue. Depending on how old the boat is, and what outboards you're talking about, the weight of the outboard could be a real issue.
Back in the 60s and even into the 70s, the 40hp outboards like the johnnyrude big twins weighed about 150 lb, give or take. The 50s were a lot heavier, well over 200 lb.
Later on, the weight differences weren't that large, so it wasn't as much of an issue.
So...how old is your boat and what engines are you comparing.
I know one thing...hanging an old johnnyrude V4 50 hp on the back of a boat rated for 40 hp will get you in a lot of trouble due to the weight.
It's not just the horsepower.