Sulli : I think I'm (unintentionally) helping you drift from the original subject title of dual exhaust. To get a more clear picture of your problem, what engine do yo have in your truck and is it stock?
My summary opinion on your quest for a dual exhaust as a solution to engine roughness --- I would check a bunch of other possible problem sources first before determining that the exhaust manifold flapper is giving you that much performance head aches. Going to duals can be expensive, separating the manifolds to clean the flapper shaft can be dirty labor (with the chance of busting those manifold studs!)
On your ignition timing question: Chevrolet has had a long tradition on those Stovebolt 6 engines of using an embedded ~3/16ths BB-like ball to set the "initial" ignition timing. They did this so the little BB thing could be spotted easily with a neon timing light. {Chevrolet engineers found that ignition timing with the engine running was preferred because it compensated for the slop (back lash) in the timing gears, and distributor gear & distributor cam. Especially on early IL-6 Chevies. To see if the distributor cam lobes were OK, #1 cylinder was first timed to the BB, then the neon light was switched to #6 cylinder: and if the fixed pointer still linedup ... dist cam was basically Ok.}
That little BB is only starting place for your ignition timing, you MUST tweek the setting a bit for the octane of your gas and your engines ambient running conditions. My 51' 216 runs (pulls hills & starts, drives highways etc) best @ ~7-8 degrees advanced from TDC. Finally, TDC is often referenced on the old stovebolt flywheel with the symbol UIC (upper line center).
Note that "valve timing" is NOT the direct same as "ignition timing". Setting the valve tappet lash on your engine is setting the clearance SPACE between the rocker arm and the valve stem. It will clatter if too loose, it can cause valves to burn and loose engine power if too tight.
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Yes, using SeaFoam or Marvel Mystery added to engine oil to try and get valves un-sticky is a tried and true practice attempt. It will not cure worn or deformed parts, it may unstick a varnished area.
