MIke,
I like Brian's suggestion of Blumlein, I'd set up the coincident pattern about 4-6 feet directly in front of the group with the drums and bass making a sort of triangle with the mic's.
If the drummer and bass player are "contained" and are careful with their dynamics, they shouldn't overpower the mix.
Additionally, you should get them to play the loudest tune/passage they will EVER play for the set up soundcheck. Ask them to really BLOW, play as loud or louder than they will play during the rest of the music. Tell them you need about 10 minutes of this and record it. then ask them to take a break so you can litsen back with no distratctions. if you hear one horn or one vocall too loud or too faiint, move them around the stage so they are closer/farther from the mics.
On first glance, this set up should result in too much Sax and Trombone, annd maybe not eniff vocals. I know the trombone is the lead instrument, so you will have to deal with that, but the other horns or vocalaists may have no problem switching places or moving more in a line as if they were in a big jazz band set up. One of the reasons the trumpets and trombones are in the back of a big band set up is because of the overall "throw" of their sound tends to be "longer"/louder than Saxes or vocals.
one last thought, i would like to see the bass and guitars switch sides of the stage, this will affect the "imaging" and loudness of the 2 instruments on the recording.