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The even/fairly decent comp readings tell you the long block is healthy, I dont like to shotgun parts to a car to diagnose so I wouldn't suggest buying an ECU just to guess & I'm thinking the ECU may be OK tho nowadays keeping a spare makes good sense with their track record for breakdowns. With the dist cap on make a mark on the top of the dist metal perimeter rim at the center of a cap terminal bulge & pick the bulge slightly to the left CCW of where the rotor is right now then turn the crank with a 1&1/4" socket/breaker bar clockwise slowly/carefully till the magnet is dead even with the next tooth then at that point see if rotor is pointing at the center of the mark you made on the dist metal perimeter or very close, which is rotor phasing & if it's very off post how far & which way (vac adv shifts it CW from it's at rest location on a big block). I'm assuming the plugs/wires are in good shape & routed 1-8-4-3-6-5-7-2 CCW. I keep coming back to a vacuum leak & it is less noticeable at higher rpm's since there's less time between each firing so that masks the problem. Myself I think I'd pull/replace the valley pan which on a BB ain't hard to do & use Permatex #1 in the toothpaste like tube lightly/evenly around each of the 8 port openings on the valley pan & check if bolt holes/ports are matching (on the slope). Maifold may not be dead flat. Good visual & go from there.

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After turning the dampener with a socket I can move my dampener 10 degerees in the opposite direction before the rotor moves again. Is this too much play in a timing chain?

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Yes & alot of that is likely in the lower dist shaft tang clearance in the top of the intergear slot.

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Could you please explain what this is?

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The tang on the end of the dist shaft is alot smaller than the slot groove in the top of the intergear that if fits into. You want it tight like a key in a lock. rotate the rotor back & forth & see what you have just from the dist & 10 deg total is too much to ignore but I would still suggest R&R the intake.

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