289 Will I notice the difference? Performer RPM

you will feel the power, then you will want more

1 7/8" headers?

I can assure you that combo will leave you wanting more on a stock motor, LMAO

You've gotta get those heads!!!!
Sell your mother or your first born or somthing like that.

I have the same setup, RPM minus heads, and this thing RIPS!! I would say its a must to run higher gears, I run 3.80's (higher, lower-whatever). And its ALOT of fun to drive. And YES, I now want the HEADS. Actually I might get the AFR heads over the Edelbrocks. But everything else...holy moly.

Oh yeah, and if I had it to do over, I wonder if a Road Demon carb. would do better than my Edelbrock. Hindsight being what it is.

What size carb are you going to use?

Next I would work on rear gears and then try to save enough to get some aluminum heads.

HistoricMustang
www.historicmustang.com/tech.html

I think you mean 1 5/8" headers?

Id do it. RPM intake with an eddy 600cfm carb. You will definetly feel a difference.

If you want the RPM cam, it is made by TRW. Edelbrock just puts it in their box.
You can get it cheaper from other sources.
Summit uses the TRW grinds as it's "house" brand cams.
I don't know the exact specs on that cam, but if memory serves, it is a bit lumpy.
If you have an auto and highway gears, you will probably be unhappy.
IMO, I would go:
Comp 270 K-kit
Used 1 5/8" LT headers (there are a zillion out there and they are dirt cheap used)
Wieand X-cellerator intake
Your 600 carb
An exhaust equalizer pipe (H-pipe)
2.5" exhaust out the back through 2 good mufflers.

That would REALLY wake up your car. It wouldn't be quite the same as a newly rebuilt engine with fancy heads, but it will be day and night from what you have now... a stepping stone to something better later.

With the above combo, you will want to get somewhere between 3.25 and 3.70 with the rear gears as soon as possible to get the best results.

Good luck
Dave