471 Big Block high rpm problems

Hi

I have been fighting with a problem that my car has since I got it.
The car is 1971 Plymouth Duster with 471 big block + 727 TF with manual valve body.
For ignition I have MSD 6AL with 7000rpm pill + MSD distributor.
Engine is simple combo with hydraulic cam (540/544 lift due to 1.6:1 roller rockers).
Timing is 18 initial and 36 total. I have one light silver and one light blue springs in the distributor. Full advance is at around 2300rpm.
Around 11:1 compression, I run it on premium fuel with additive.

Here is what happens:
When car is in park or neutral I can rev it all the way up to 6500 no problems.
It responds to gas pedal very quickly and starts great cold and warm.

Problems start when I drive. It seems that car is loosing power and starts sputtering/misfiring around 4500-5000 rpm on every gear. Up until this rpm range it really is quick and responsive.

Here is what I already did:
1. I swapped 911-16 Comp springs with 935DR Isky springs (I was sure mine are failing, new are a bit harder also)
2. New spark plugs (NGK with 6 heat value)
3. New spark plug wires
4. Shielded MSD wire between the ignition box and distributor
5. Checked carb tuning with AFR sensor
6. Checked carb and intake for air leaks

All these things changed nothing.
Yesterday I started backing out the distributor and checking for difference in behaviour. Turned out that when I backed it to 8 degrees initial and 26 total my problems were gone, but I could only rev it to under 6000, also felt like I lost quite some power.

H E L P, I am so frustrated by this!
Any ideas?