Which dynomax ultra do I need?

Agreed. There the same as (or very close to my) Hooker Aero Chambers & Dyno Max Super Turbos. They will fit right in that area that is literally under the seat.

The question you asked is really a question of muffler case length. Not the overall length. Just the case. The tabs or pipes that are on the muffler can be trimmed down so the rest of the exhaust piping can fit into it.