I have a 69 SS 350 that has a 350, T-350, and 308 gears.
The 350 has an Edelbrock Performer intake, and headers.
Cam, and internals, are unknown. I have a Q-jet on it now, and it runs
verry well, but I have been told that it may perform much better with
a carb change, and possibly a cam change. I am looking at
Holly Street Avenger 670, and the 4160 600cfm. What would be
the best way to go? Are there other carbs I should consider?
I thought I would leave the cam alone until I try a carb swap.

if the qjet is in good shape..ALL the ol'skool hotrodders will tell u to stay with that. If your valvetrain can handle it, they new comp cxam xe274 or
xm278(<) is great for the street/daily driver car....JMO tho.

I have a Q-jet on it now, and it runs
verry well, but I have been told that it may perform much better with
a carb change,

If it ain't broke, don't fix it.

agree, nothing wrong with a properly adjusted q-jet.
If she's running out of breath too soon, I'd go with a perf rpm or air gap intake but keep the q-jet.
Standard performer manifold is for truck motors!

Well what are your goals.Do want to add a different cam?
With those gears you can't get too big there.

Ditto on keeping the Q-jet for now. (and this from a die hard Holley guy)
On a base level small block the for the street the "Q" will give you better throttle response and better fuel economy than the Holley because of the smaller primaries. But when you open it up on the highway the larger secondaries are there for the oomph when you need them. With an unknown cam it's hard to say. What is the problem? If it's sluggish on the bottom end then the cams probably too big. If it just doesn't want to go after 3k then it's a stocker and should be changed for performance use. Comps xe262 is good for mild performance, gives good response and works well with your intake, carb and gears. But if your looking for a little power and don't mind a lump in your idle the xe274 is a great small block cam, but you would want to change the intake to a high rise duel plane like the Air Gap or Stealth.

Agree,..keep the q-jet. I can't imagine a 670 Avenger out-running a well tuned Q-jet (which flows 750).

Do the taller intakes, like the Air Gap loose verry much on the low
end? Also how is hood clearance? I have a cowl hood, and 14 inch air
cleaner. I was just thinking of ways to get more power untill I build
either a 383, or big block 402,whic have sitting in my shop. New
motor is waiting because this one runs great. If it aint broke!

Well a Qjet and stock manifold will be better on a very mild combination from off idle to about what 1500-2000 rpm.Above that there is no loss.

So far from the input on the boards it looks like the Weiand Stearth Air Strike is a better power producer than the Performer RPM Air Gap.I'd sure like to see a couple people get one and try it in place of theri Air Gap to be sure.A couple engine builders have seen the Air Strike and have reported that they look like a nicer casting on the outside.

There will be no clearance issue if you have a cowl hood.