Stephen, check out the you tube clips of Vortex mufflers. There are several, including V6's, and most are single muffler systems. They sound great without extra resonators.
I'm not familiar with the Thunderbird exhaust, but I think you have long tailpipes. If the tailpipes are not really short, the upstream pipe sizes will have no affect on drone etc. The muffler creates most drone, anything else happens in the big ricer jap car end pipes. With normal tailpipes, you may be fine without the tip mufflers you mentioned. I'd try that first.
I have a V8 SUV which I just received two of the Vortex mufflers for. I like a subtle sedate sound, as low as possible. My exhaust will be twin 3" cat pipes into twin pairs of 2.5" pipes, into the Vortex 2.5" inlet pair mufflers, out of single 3" pipes. You can't do that under a car I know, but this will fit easily in a truck and create a natural crossover.
The Vortex mufflers measure just over 4" thick, welded together they will be less than 8.2" thick. That matches my stock single muffler with its shield.
91 Mark VII LSC SE, Griggs and MM suspension soon
98 Mountaineer, A4WD, Big brakes - 12.75" x 1.25" rotors