You certainly will not NEED a tuned system, and that could get longer than the space you have. The canister mounted in the belly pan, centered on the CG, will remove the need for both the battery to be aft, and the lead under the stab. Mounting the battery at the CG will get it very close.

Another ST detail - the hardwood beams need just about 1 mm removed from each to be wide enough for the ST to slide on in. The header will eliminate the need for the muffler spacer, too.

Please remember that this is unflown with this combo, but my earlier flights (and many others) have validated that it is a very capable design.