360 engine vibration at ~3000rpm

Guys(Sorry for the long post, but this is a 3-year story),
Have read content of various posts about internally and externally balanced crankshafts, etc.. but still have a question about my engine vibration & Noises. Situation:
-Bought a 74 duster a few years ago in pretty rough shape..for a fair price.(Bad vibration at 3000-3300rpm, slipping 727 transmission, but "rebuilt" 360 engine, solid body/great interior, 8-3/4 rear, 4-wheel discs; Fr&Rr sway bars, frame connectors, Hotchkiss rear leafs, ). Knew I had some work to do, but the "bones" were good....
-During trans rebuild found: broken 1 bolt btwn flex plate to torque converter, 1 broken flex plate bolt at crank, bad torque converter, and various internal damage to trans.
-After trans rebuild w new correct flex plate, new correct hole-shot torque converter, and all bolts....Vibration was still present ~3000-3300 but MUCH, MUCH reduced.(more like a bad exhaust vibration but still of a concern.)
-Factory style harmonic balancer is in-place and appears to be O.K.(Have not replaced it yet.)
-Since purchased, the engine has pulled strong (mild cam, long headers, edelbrock intake, 680cfm AFT carb..) but always had a loud tick(or low knock) under acceleration that many pals have diagnosed as a header leak, engine ping, etc... but was never loud enough to be thought of as bad rod/main bearing, etc..(Also never found any metallic "shiny bits" in oil.)
-TROUBLESHOOTING THIS SUMMER:
a) have slowed timing till engine barely idles smooth and mixed about 15% race gas into tank to eliminate possibility of detonation "pinging". AND: Noise still persists under acceleration but not when cruising down road at 45-50 on a flat stretch.
b) Gone through header/exhaust seals & everything is tight w no leaks
c) have gone through the rocker arms and replaced several to minimize any "clicking" as an aid to help identify the louder "knocking" sound. AND: engine is quieter but knock still persists.
d) installed fresh 20W-50W oil(have been using 10-40W syn. w Zinc add.) oil along with Lucas Oil Conditioner, AND: noise persisted w no changes.
OVERALL....Engine starts and runs good and can be revved in neutral to 2500-3000 rpm with no vibration or "knocking". I have not driven it "hard" and rarely go over 4000-4250rpm)
SO,..Am I kidding myself and simply have early signs/sounds of a failing rod/main bearing? Does the vibration at ~3000-3300 signify a crank imbalance or a bad harmonic balancer? Any other troubleshooting ideas before I start shopping for a crate engine?
THX-Scott