I've built and flown the GP .60 size Cub. I believe the .40 has a little different design. If you can afford a 4-stroke, go for it. Cubs beg to have a 4-stroke. Any .60 to .70 size 4stroke would be a good choice. The Magnum .70 would be good.

Since I haven't built the .40 size, I don't have a whole lot of tips. I would recommend waiting until the engine is in to finalize servo and radio placement. I glued my servo tray in the location on the plans, but ended up having to add a lot of nose weight to balance.