Dan,
If you are determined to go with the BVM powerplant there are two ways to address your pipe concern. You could heat up and bend the BVM pipe. That might take a few iterations to get just right.

I had the same concern with asymmetric thrust that Shaun mentioned on my F-4. The pipe blocks most of the area of one of the exhaust cones, so most of the thrust comes out the other side. You can offset this with a bit of rudder trim. My solution was to use a Weston Mini-pipe.

The Weston pipe is about 9 inches long so you do get exhaust residue in the rear ductwork, which is not a big deal. The performance was very good with a BVM 81 engine. I have no experience with a BVM 91 and Weston mini-pipe, but it should work fine. If you want to try it I still have the Weston pipe and can give you a good deal on it.

Joe