Lower the lid and tilt the windshield back? 'Bout the only thing that's going to help lower your frontal area...
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One of biggest things hurting you is probably the deep inset grille, the hood catches air and creates lift which not only makes the front end feel light at really high speed but also lowers the efficiency. Pretty much an issue with most muscle car era designs. Unfortunately, there isn't going to be much you can do about that. Adding a front spoiler helps with the lift, but that doesn't help with the drag that you want to reduce for better mileage.
Look at what Dodge did for the Charger 500- flat front grille, lost the recessed rear glass. Both massive aero improvements. More on that stuff here
https://www.roadandtrack.com/car-culture/classic-cars/a29285/dodge-charger-500-nascar-history/
Getting ride of the deep inset grille was one of the reasons the '72+ Challenger got the "frowny" grille, it was set much further forward and was a significant aero improvement vs the '70 grille
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The other stuff is little, tucking in your bumpers helps some especially at higher speeds, that was another NASCAR trick- they'd fully smooth them into the bodywork. And even on the backside they covered up the inset bodywork. Obviously this probably isn't something you want to do to a street car, but if you wanted to see if it would help you could just tape those gaps on your car and see if there's any noticeable improvement.
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You obviously missed the part where he said he has a T56. As in, a 6 speed transmission that has not one but TWO overdrive gears. 6th gear is either .63 or .50 on that transmission, so the 3.55's aren't hurting him at all. He can probably cruise at 70mph at 2k rpm or less. Hell I run 4.30's with my T56 and I'm doing 75mph in 6th at 2,500 rpm.