I was wondering at what speed is a wing needed on a 71 Dart? I run 1/4 mile. The car will be lowered 1 1/2-2" in rear and the front will be lowered so that it has a slight rake at rest. Thinking of a front spoiler also. Leaf springs, radials, and hopefully around 3200 lbs.

I think it will lower the CD of anything and possibly help reduce lift on the rear, but I don't know what speed draws the line on NEEDING one.

If the car gets light, sensitive, on the steering inputs or feels like the front end is floating around the last 300 or so you need more down force
My 1971 Duster ran 134. MP in the 1/4 mile and it was stock front and rear with a slight nose down stance, no issues at that speed
My new car, 1970 Cuda, runs a 147. MPH in the quarter and it has a wing on the tail behind the deck lid with zero angle in relation to the ground and it doesn't move around or feel like it is floating. It has no front rake angle at all, it is almost tail down I didn't build the car( coil over suspension, front and rear) and there is no way to raise the rear without redoing the 4 link mounts, which I'm not going to do as long as there are no issues with the handling
Fix it if it is needed, don't if not

What I have learned from the track and the
wind tunnel guys is this... the deck lid on
the car is the big factor.. if its longer then
it will have more down force.. on my car with
135 MPH was a breaking point... the back of the
car would start to lift and I couldnt put ANY
steering into it to be stable.. I lost control
at 135 with a quartering headwind... after I made
it back to the trailer I put the car away... on
Monday I talked to the wind tunnel guys and seen
smoke footage of my ride.. the air was dumping down
about 1 foot past the tail end... after 2 designs I
came up with a rear spoiler/wing with a few degrees
of angle on it.... it was day and night different and
drove GREAT.. plus picked up 3 MPH... this has 5 spill
plates(so air doesnt slide off the ends)

I'm thinking spoiler then, not a wing, on the back. And a Camaro chin spoiler on the front.

So far I've gone 160 with stock steering and it seemed to handle fine. It'll have a Bob's Profab front end in it the next time it goes to the track.

That looks great. If I could pick up 3 mph with a rear spoiler I'd do it for sure.

Just make sure it has at least spill plates
at the ends or yo will loose a lot of effect

I forgot to mention that the Cuda does not have a front spoiler, the car had a blown B1 motor on alcohol in it that ran in the mid 7.80a at 184+MPH according to the former owner, that is the reason for the rear spoiler on the car.
I removed the parachute and the 72 inch long wheelie bars from it so it fits in my trailer better.

A wing or spoiler always increases cd and drag. Although it may be in the car's aero "shadow" (frontal cross-section) it's also generally more frontal area since it's partially in clean air.
More nose down angle = more down-force and drag.
The effect is also highly dependent on deck lid size and shape, height of the wing, etc. Depending on the back light shape, sometimes a wing won't work at all until it reaches a certain height.
A actual airfoil has more down-force per lb. of drag than a simple flat plate, but of course slightly heavier and $$.
The spill plate is also an important safety feature. If the car yaws at speed ("fishtail") the spoiler becomes effectively shorter, and down-force falls off resulting in oversteer. The plates help keep the air on the "foil" part.
What's difficult to plan or estimate is the relative effect of front vs. rear devices. If they don't have the same proportion of down-force and work at the same speeds the front-vs-rear balance will change as speed increases.

My Dart is rock steady and easy to drive at 150+,no wing. Every body style will react differently.ask around about your body style. My Dart has a lot of rear deck lid area, which I think is a good part of the excelent handling.