Vibration ONLY at 75 mph

I have and interesting issue with my 65' Dart, 170 two door Sedan daily driver. First of all I am am a new member to Fabo, but I do post on Fbbo once and a great while for the shop I work for. I'm looking for educated answers on my problem, so please read all my post before answering so there wont be any confusion.

My Dart is a factory 4-speed car, with a mild; 225 s/6, (stock low milage short block, ported head/larger 1.7/1.44 size valves, that's been ported and milled, shorty clifford headers, Offy 4-bbl intake with a 600cfm edlebrock (AFB wanna be) carb and a Comp 264S cam. The trans is a 3.09 first gear non-o.d trans, that has the 68-up 30 spline main shaft and later large out-put shaft tail housing. The car uses a stock cast iron flywheel, and stock replacement 9-3/8 clutch disc, and a Chevy S-10 2.5 four cylinder pressure plate. The clutch was balanced prior to installation. The drive shaft is a custom built unit with 7290 solid Spicer u-joints front and rear, connecting to an A-body 8-3/4 axle with 2.94 gears and a Sure Grip (cone clutch style). Pinion angle for street use is at negative 1-degree in back (pinion points down ward in relation to the drive shaft) and zero at the front. The car also has 10-inch A-body brakes all the way around, 340 torsion bars, custom made six leaf springs in back (with new poly bushings, and new gas shocks all the way around. The front suspension has been rebuilt with all new poly bushings, and Moog ball joints, tie rod ends, idler & pitman arm. As of now, the car has a fresh trans, diff, balanced driveshaft and freshly balanced 14-inch tires all the way around.

The problem; ever since I put the car together over a year ago there has always been a slight vibration between 72-77mph, any slower or faster it is a smooth as glass. But at 75, it feels like a driveshaft issue, enough to realize that there is something is not right. The vibration can be felt through the entire car; steering wheel, dash, seat, floor, roof, sun visors, etc. Accelerating, or deceleration doesn't change it at all, and neither does driving with the car in gear propelling itself or even coasting (trans in Neutral, with the engine at an idle)the same vibration is present. Since the car began to run, over the last 40,000 miles and almost one year of driving I have changed the following:

-Tires, front and back
-Wheels, front and back
-Driveshaft, (had two different ones) and the current one was just balanced
-Complete trans
-Rebuilt the entire front suspension, inspected all parts for wear, play, cracks etc. Installed new front shocks and t-bars
-Rebuilt rear suspension, new leaf springs & new bushings, new shocks
-Tried a different pair of SBP axles
-Tried a different pair of rear drums
-Tried a bunch of different driveshaft angles
-Complete drop out section of diff, including different flange, case, gears, sure-grip unit etc
-Tried a different pair of front brake hubs and drums
-Tried a different clutch assembly all together (first clutch set up slipped while power shifting)

Since the vibration isn't terrible, I just usually avoid driving at that speed. 70 its perfectly smooth, and at 80 its perfectly smooth. After everything driveline and suspension has been changed the vibration has not gotten worse, better or different it remains the same. This car has been very reliable, as it has been driven from Southern California to Northern Oregon (1,000-miles, in 18 hours of straight drive time at 70-80mph) three times, and it has gone on at least two dozen 300-mile drives with ease. What has me puzzled is that everything but the engine and the body has been changed over time, one part at a time and nothing at all changes. Again, to clarify; the car does vibrate at 75 mph coasting even if the engine is off, engine rpm and/or load has no effect on it.

This car when it first got running, got a bunch of drag strip use after I already was aware of the vibration, with lower gears the vibration was still present at the same traveling speed, regardless of how fast the drive shaft is spinning, from 4.10's to 2.94's still the same. The car endured over 100-drag strip passes, clutch dumps, wide open throttle power shifts etc.. The problem again never changed..

So if you actual read all of this, please any insight as to what it could be would be greatly appreciated. Thanks in advance, and I've loved getting to read others post on here for years, and its great seeing all of the enthusiasm for these cars is still alive and well.

Btw: Here's a picture of my car at Wildcat Auto Wrecking up in Oregon on one of my trips up north, most of the paint is original lol..