As long as the 4" stacks are just for cosmetic effect, and you use small-diameter pipes off the manifold flanges, the low-end torque shouldn't be affected much. Run pipes about 1 3/4" or 1 7/8" diameter from the manifold to the stacks, and adapt them to the larger pipes for visual effect. Any semi-low restriction mufflers will produce the sound you want, guaranteed to give you a headache if you drive it very far.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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