I had a '99 Suburban 350 that I converted to true dual (easy because it used a 2 into 1 muffler). At first they sent me on my way without any crossover tube. It felt sluggish of the line, like I had lost power below 2500 rpm. Went back, had 'em weld in an H-pipe. The most convenient spot was at least 6 feet downstream of the engine, maybe more. After only a few seconds behind the wheel I could feel that all the lost power was back and then some. Also the interior resonance was lessened. I think a crossover anywhere upstream of the mufflers is better than no crossover at all.