I think the 4*40 with a 91 4 stroke is, not only going to be grossly overpowered, but very nose heavy also.

You should be ok with the 4*60 and the Saito 91 or 100 4 stroke. Just beef up the firewall area some, and try to keep away from high G manuvers (ie the Wall) and the plane should hold up for you just fine.

What you do want to keep in mind before spending the money is, what size planes do you plan to fly in the future ? If you are going to be flying primarily 60 size planes, the 91/100 will be a good choice, but if you are going to be flying primarily 40 size planes in the future, I would recommend the 4*40 and the Saito 72 combination.

Hope this helps.