Easy (?) OD Auto Install

Yeah, I agree; 75@4000 is too much for me. 3000 is even barely tolerable.

I have very limited experience with BBs and zero with anything over 440 cubes.
With your stated combo, the math goes to 75=3688@zero slip, could be|
up to 3800 @3% slip on the tach.
4000 would be 8.5% slip,
which points to; a slipping/loose convertor, a tach-error, or 3.91s actually being back there........

The 200R4 has ratios of 2.78-1.57-1.00-.67od
The A727 has ratios of; 2.45-1.45-1.00
So, with the 200R, your Zero-slip@75mph comes down to 2470. Adding 3%slip for a tighter GM convertor, that comes to 2545, which is high enough to be able optimize the timing for, using a factory Mopar type distributor, especially if you have alloy heads, which in my modest experience with my 367, don't need quite as much timing advance as iron open chambers.
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My 367 with alloy heads cruises best at a higher coolant temp. I surmised that was because the alloy heads were extracting heat out of the chambers even as it was being produced. So I painted my heads in an effort to keep the heat in, and jacked the minimum coolant temp up to 205*F, and shazzam, with optimized timing, she was up in the mid 20s mpgUs @65mph, which is the speed limit everywhere in Manitoba, except the TransCanada hiway.
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But I gotta ask you something;
You have a lightweight Dart, with a Monster motor, and stock narrow wheels, with, I assume tiny tires; that should roast any tires right up to cruising speed.
Have you thought about just running less rear gear?

I mean, 3.73 x .67= 2.50 equivalent. and/or 27inch tires over 25.5, is the equivalent .66 gear reduction.
Say you keep the 25.5s, and get a tighter convertor that cruises at 3% slip.
To cruise at 75=2550@3% slip, would now require a rear gear of 2.50..

>> 2.94s, 25.5s, and your current loose convertor would get you 75=~3130
Your starter gear would be 2.94 x 2.45= 7.20, but at zero mph, that number could easily be multiplied inside the convertor by say 1.8; so then, 7.20 becomes 12.97, which is killer. This will ramp down automatically over the next second or so at WOT, to become something like 7.2 x 1.4(estimated.)= 10.08 at the top of first gear, which is still a killer number. Second gear becomes around 1.45 x 2.94 x 1.3(est) in the convertor= 5.55ratio; Which will get you ;
6000@ around 103 mph with 25.5"tires. (still in second gear).

For a lightweight streeter, with skinny tires, and a stockish back-half, these are very nice numbers; and that BIG engine should have no trouble pulling this combo.
EVEN if you mini-tubbed that Dart, and kept the current loose convertor; you would still need a better rear suspension to keep from blowing the tires away.

How much is a 2.94/SG pumpkin gonna cost you?
Unless you run a potent traction-aide back there, you can even stick a small-pinion chunk back there. I mean guys used to give those away.

For a lil more money; 3.23s, 27s, and a tighter converter, would get you
75= ~2900; and at that rpm, you can easily tune the ignition system to optimize the cruise timing, to optimize your mpgs.
Altho,
with 505 cubes,
not sure it will make that much of a difference, lol
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Now, if I had 505 cubes in a stock-tubbed, street-Dart;
I would think about putting a tiny 228ish cam in it, and gear it with a Loc-Up overdrive, to cruise 75@ say 1700
which would take that 200R4, and 27s, and 2.76 gears. BUT
I might put a bit of stall in her, just for a bit of, off-the-line fun.
With 2.76s, and 27s, and a 2.78 low gear ;
first gear will go to 50mph @ 5200ish/ 60@6250ish; Bam!