To me it sounds like the diaphram in the Carb. is getting stiff.

Runs good one time and not the next.

The most you have got out of this engine is 7200 and now you are saying it's leaning out at 6000rpm.
By the way adjusting the low needle as little affect on the high needle byt the high needle does affect the low needle.

There is something wrong with the new carb or your tach.
What numbers are on your Carb? Especially the butterfly.

You did not address my question about the compression.
"Does it feel like it has good compression?
Can you turn it over easily by hand, or does it get real hard to turn when the piston
comes up on the compression stroke? "

A video would be nice. You can only judge so much with words.
Sight and sounds are really helpful.