I feel like I am giving up some mph. The ET typically is better than the MPH and I know that it is not a horsepower issue this bread Box does not go through the air Politely... any and all information comments and sarcasm welcome

There are three schools of thought on this:

A wing, or any similar shape even if it's part of the bodywork, trades speed for rear wheel traction at high MPH (nothing significant below 100). When parallel to the air stream coming off the roof (probably not horizontal) it adds (almost) no downforce but still adds drag and makes the CD worse. A sharp down angle on the nose will effectively increase traction and slow up the car a lot.
If you have no oversteer or non-linear RPM at speed, a wing will only slow you down.

Without major bodywork, have you considered what is legal for the nose?

I run almost 150 mph 69 dart ! the wings I looked at I would need a longer trailer 20ft with the tool box wont work. My friends top sportsman 7 second Chevelle made additional mph by damming up the way the air hit the firewall and directing the air I don't have inner fender wells not sure if that helps or hurts air speed! maybe others can advise from experience .

One of the top speed SCTA racers at El Mirage dry lake told me years ago that anyone can run 125.MPH, a lot of guys can run 150 MPH
But to get the same car that runs 150 MPH to run 175 MPH you will need to either double the HP or slick up the car and chassis to reduce the wind drag.
I seem to remember him saying about 5% reduction in drag would do that for you on that car in his example, a full size door car, not a roadster or lakester

I put a small wing on my car and it made a world of difference.. I was loosing traction at 130mph and I
ended up loosing control of the car(multiple times) I went back to work and talked to the wind
tunnel guys... they gave me a few ideas to try.. I still have that wing on the back.. car drives like
a caddy now.. never had any more issues.. when I put the wing on it picked up 4mph with
zero other changes.. I gave it 2* tilt upward and long enough past the back.. this was all
trial and error but worked great.. I also put spill plates on it to give it more strength.. I am
running 152 now and its smooth as silk

If you have fab skills, make your own. Make it full width, about 14-18" long and keep the surface level with the decklid. Spill plates that integrate nicely with the quarter panel and about 6-8" tall and a few inches past the end of the wing. Lastly put an adjustable wicker bill on the end. Make it be able to go from 1 1/2 tall to 1/4" tall. You will be surprised the difference it makes with different height adjustments. I don't have any good pic of the wing on Larry Boyd's GTX but its a perfect example. I can get some if you like. You can also put lexan behind the grille. That area catches the most air.

You need a spoiler not a wing. A small wickerbill and decent spill plates.
Jerry Bickle has a nice carbon piece that's reasonable and easy to fit.
The car will drive better.

NICE DART!

We had this one 'back in the day'...

It was good for 180mph, more of a pro-stock kinda deal?

I think you need to stop the air 'spilling over the back', maybe even a modern day Nascar one would work?

A 69 wing might work if you is going 200 lol...

Sorry I couldn't resist as I am a Nascar NUT!

They had ones like you are thinking about, a bit ricer?

Or just a 'good old fashioned' single FLAP.

Front Bumper looks very high to me?

Lower the whole grille and the bumper by two to three inches would help...

Going over 150 means a CHUTE, more grief...

Yeah Bob, Could we get a couple of pics? This is along the lines of what I was planning to add.

The original question is NOT "how to make my car handle better?", but "how to make my car reach higher MPH?".

A wing parallel to the deck lid may be exactly at, below, or above the air coming off the roof. The wing height above the deck is another variable. This will also change as car speed increases. There is no easy answer except a wing slows the car down.

At speed, a few things are happening generally, and especially on a Dart of that era:
The recessed grille is packing whatever air doesn't get through (which also rams into the compartment & firewall).
(Venting the hood is commonplace among front-engine road-race cars, and now, the next-gen Cup cars.)
Air getting under the car is bouncing off all the undercarriage stuff, especially the ft suspension.
Air going over the roof becomes detached at the back glass & trunk lid, and roils (think an eddy in water), with the side-effect that it essentially pulls at the rear of the car.
A spoiler or wing essentially pulls the air more smoothly over the back glass & trunk lid, so that it leaves the car less disturbed with less effective pull rearward.
The Dart back glass is kind of a mini-version of the 2nd-Gen Charger, which had drag issues from the disturbance.
The idea is to get the air to go smoothly around the car, not through it (except for cooling considerations) or under it, and to depart smoothly behind it.
Spend some time watching airflow studies, or even water moving around rocks, and you'll see.

Fun fact: Adding a 4" angled spoiler to the back of the Melrose Missile picked up 7 MPH when it was stuck in the 150s.
The aero effect in that case more than compensated for drag.
Same thing added to a PS Camaro would make it more stable, but it would lose MPH.

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Here's the one Mike put on the Valiant when he was spraying it. He said it drove weird above 120 before he put it on.

Here's the chin spoiler he used. He also made block out plates for the grilles.

I would watch rear shock data to tell me what the wing is doing...

None of your pics are working for me Slant.?