340 No low end power

Always start with timing. run as much initial as you can with out it kicking the starter or pinging. Then you need to limit the total timing to 36*-38* at around 2500 to 3000rpms. Im not sure how the unlite works for adjusting total advance. The springs will effect how fast the advance comes in.

If the Carter you are running is an after market one its the same as an edelbrock. Check for numbers on the carbs base near the front passenger side mounting bolt.

Also, Im not familiar with the Autolite 66s. Ive always ran 65s in my small blocks. Anyone know the difference ?

See if you can tune what you have before spending money.