I fly flat bats all the time. Mine have Norvel .25's and MDS .28's on them. I fly "B" class combat and they are very combetative.
A 6 oz, tank will fit inside the bat. Just cut a hole in the center of a Dave Brown mount for the fuel lines to exit the fuse. Stuff a little foam under and on top of the tank.
You didn't say what type wing you have. I fly a 48" RNAF dogfighter wing. Balance is 3" back at fuse.
If you fly a flat bottom wing, make sure and "reflex" the ailerons up about 3/16". This will help considerably with trim.
The "Flat Bat" is fast with a .25 or .28. It would be very fast with a .32. Most .40's will not fly as fast as a good .25 or .32 because they do not turn as many RPM's.
If you keep the weight to around 3 pounds all up, the bat will turn as tight as anything out there and looses very little speed in the turn.
Hope this helps.
Gunfighter