Rebuild or Replace 850 TQ for a Mild 340?

TQ all the way. One of the best street/strip carbs ever made, bar none.
Check it out well, freshen it, and get it set up right!
Thermoquad Teardown and Reassembly How To
A TQ 9009S ('75 440) would still have a vacuum advance port, will work just fine on a 340.