Just a little miffed...

So this is one of those things that I can only hope im being too critical about. Im a little worried if i have an exhaust valve/seat taking a dump, or a spark plug issue, or something else. Here's the scoop, 87 vintage 318 highway patrol motor, 360 top end (factory) 110,000 miles on this monster. true dual exhaust 2 inch head pipes that turn into 2.25 inch approx 1.5 feet after the manifold flanges. msd 6A ignition, high vibration coil, napa belden 8mm red wires, aluminum tower cap, NGK V-power plugs, 91 octane no ethanol fuel. 36 degrees advance at idle (including vacuum) holley 4175 650 cfm spreadbore. 18 in. lbs manifold vacuum at idle, very consistent, no needle jumps whatsoever.

Heres whats going on, ever since i got this beast rolling earlier this year its always had this weird sound on deceleration, this is going to sound stupid but i dont know how else to explain it, its a sort of poof sound, instead of the nice rumble it has this intermittent poof/sputter sound more prominent out of the drivers side. Ive ran champion copper core plugs and now NGK V-powers, same thing happens. The thing runs like a raped ape though, but this has been driving me nuts. I pulled the plugs tonight, NGK's, the majority were a little black, honestly from shifting too fast under load (running really fat) because the original kickdown arm on the trans (for this drivetrain) was hitting the floor so i ran the shorter slant six 904 arm until i fixed it properly. I have since then modified the floor to clear the correct arm so the shift points (and lockup) are right where they should be. Ive messed with the idle adjustment screws until i achieved the highest manifold vacuum and idle speed. putting my hand over the end of the pipes i cant feel any sort of suction effect on the drivers side, even did the paper trick to see if it would suck it in, thankfully no such thing happened. Passengers side is nice and solid on the exhaust, nice strong exits, the drivers side though has this intermittent weak exit. its solid poof poof poof then it has a super weak "poof" then back to being solid until it does it again, but it NEVER has a weak exit twice in a row, usually every 10-20 solid exhaust strokes. This only happens at idle, and deceleration. Under heavy load it pulls nicely, part throttle, same thing. runs great. Ive taken my timing light to see if i had a wire that was being stupid and 1, 3, 5, 7 all hit throughout the entire length and i cant see any spark jump from the boot, wire, or carbon tracking on the cap at any RPM. Cylinder wires 5 and 7 are separated sufficiently to prevent inductive crossfiring as well.

so maybe, does anyone have any sort of idea what could possibly be happening? or am i just being too critical? sorry for writing a damn novel. Thank you for any input.