Holy mother of ***, that sludge buildup is fantastically EPIC! The 56 Corvette I had that sat for 35 years didn't look even half that bad, wow!
I would do at least a minimal R&R on it personally (AKA: welfare rebuild). Get everything cleaned, inspected, hone the cylinders, new rings/bearings, and call her good. In addition to the new timing chain/gears of course.
http://www.TheFOAT.com/92GTA 1969 Pontiac Firebird w/BP 461ci stroker kit, 670 heads & XE274H cam. Primer black with black interior. 1992 Pontiac Trans Am GTA w/SLP Performance Package. Dark Jade Grey Metallic, grey leather, T-Tops.