I've got an ST .51 in my mid star, and there was more than enough room to move the servo tray and battery to get it balanced right. While I haven't tried a .70, I bet it'd work just fine and balance by simply moving the battery back a little. It is going to be a tight fit though. Getting the engine bolted down to the engine mount might be a little tricky with so much engine in a .30 - .40 sized engine compartment.