I have the 3.5 TH DP and dump also. That is why I was asking if any one made a 3.5. If I was you, I would go with the single shot exhaust. For the best flow, it is good to keep the same cross-sectional area. Having a 3.5 DP into duel 3 would slow down the exhaust flow. Plus the 3.5 the single shot exhaust weights less!!! But if you like the look of duels, I would go with 2.5. Area (one 3.5) = 9.62 in. square, Area (two 2.5) = 9.81 in. square, Area (two 3) = 14.13 in. square.
Jason