I saw one in JetPower, I think, they SAID they had no trim issues...but I have not seen it fly, myself. In theory, you can get the angle right on the turbine and just about cancel any issues out.
A twin is a totally different story. 70" is not that big, and a twin is going to be much heavier and much more complex than a single. I'd say twin if it will be MW44 type engines, or if they plane was BIGGER, 54 class engines. I know you like overpowered airplanes, though!
I was looking at one of those GIANT A-6s hanging in a friend's shop the other night. Has not moved in years. My friend is losing his shop space, if anybody is interested in a HUGE A-6, I can ask about what the deal is to buy it...