Struggles after 4900 rpm

My Duster usually shifts from first to second gear at around 5500 rpm, and chirp the tires.
Now it shifts at around 4900-5100 and wont chirp the tires. It feels like it it struggles when it reaches 4900 rpm and don't want to rev more. There is no pinging or destructive sounds, it just feels like it hesitates. Before it would pull really hard on the higher revs.

My combo:
1965-273
727
340 Cam
Edelbrock D4B intake / Edelbrock 600 Performer Carb
Headers
2.5" Dual Exhaust
8 3/4 - 3.31 - SureGrip

The most recent things I have done before this problem started are:

- Changed the plug wires. Had a used set with better lengths lying around.
- Moved the coil a little further away from the dist on the firewall.
- Bumped up my initial to around 18-19 btdc, not having timing tape I’m not sure what the total was, but it did run very well with no pinging.
Set the idle mixture according to Edelbrocks manual

I will say that it did run excellent for a while after doing these things. So once again I ask the experienced people of this great forum for advice before I go spend money on plugs and wires etc....](*,)