Ed,
As you know, I built mine with ST 45's just to see if it would work and it does. I am running 12X4's for more thrust. Take off is longer than I usually go for, but..IT WILL FLY and climb GENTLY on one engine. We wanted some pictures of it in flight, so naturally one engine dies..what the heck, let's try to see if we can keep it in the air. Down for a low pass and SLOWLY up and turn into the running engine. Level and sustained flight was accomplished with a good dose of rudder. FUN at such low power settings. No, but would it fly like that yes,,,,,no,,I am NOT going to try a single engine take off,,,not with 45's. Yes, it will roll single engine..Loops..Just call me Wuss!!!
Now with two engines..Take off is longer, as I said, than more power, but loops are flawless...Ok Ok as I can do. Rolls even. Horizontal figure 8's no problem..WITH rudder to engine mix knife edge will hold, but barely. Have not tried the spins yet. Fligth speed as you can imagine with 12x4's is not buring up the sky, but it does have pulling power.
My two cents..go with 60's, but if you have a couple of strong 40's or worn 45's it will fly rather well.