The problem with modified cars is the same as that for modified engines. When something's been modded there's always something that a potential buyer sees as needing "correction". One of my 340s was a fresh rebuild with something like a .660"/.680" solid roller cam. That cam's still lying on the bench. It was just way too much for what I was doing. I've bought a couple of cars on which the wheels couldn't be replaced quickly enough. It's a taste thing, and nobody has exactly the same tastes as another. In my own case, the last nine cars I bought were all either stock or very close to it. I don't want to pay extra for someone else's "upgrades" that in my eyes require undoing.
Regarding the receipts: While they won't determine the value of the car, they will do a lot to ease a buyer's mind in regards to the negotiated price. That price is going to depend a lot on how fast/badly you want it gone or need the money, along with whatever offers you actually receive. Obviously, the wider the exposure the greater range of offers you'll get (see below). But you should absolutely have a "bottom dollar" number in mind before you even list it. Even if your bottom dollar is $15K, start at twice that and look at anything above your minimum as a bonus.
I know the horse is well and truly dead, but another thought on the misplaced black: To me, and many others in the Mopar world, it screams "cut corners". When I see a musclecar with the bumpers, window trim, headlamp bezels, mirrors, et al painted black I immediately think, "This person refused to spend the money to do it right" which is immediately followed by, "Where else did they save a few bucks? Rod bolts? Body prep? Dollar-store oil?" That mindset absolutely extends to the engine bay and trunk. I realize in your case it was simply a beginner's mistake and truly hate harping on it, but it's probably the worst one a fella can make. But wait! You have an out here: Tell them your previous three projects were C-bodies and you were so caught up with those, you forgot. OK, that only explains 3/4 of the engine bay and not the trunk, but it's something! Seriously, as you point out, GM guys it won't find it an issue because it's what they expect anyhow, and some people just want a cool old car and won't care.
Take it to a lot of local all-makes shows and cruises--spend some time and money advertising it in person. Drive it like an old lady, but blip it in neutral now and again. Stay away from the burnout contest! Shows, especially those with decent swap meets, are where you're going to find someone looking to spend money that'll trip over themselves to do so. Hell, don't even put a "For Sale" sign in it. I swear I've had more (and better) offers on cars that weren't for sale than I ever had trying to sell the damned things. Insisting "it isn't for sale" simply drove the price to the point where I was signing the title and shopping a new ride.