thermoquad help
More than likely the jetting is too small for the 400. What #'s are stamped on the driver rear corner of the base and what #'s are cast into the top of the carb?
The fact that the carb is backfiring and the only adjustment that made a noticeable difference directly affected the jetting leans towards too lean a jet for the engine.
This info will make clear a lot of the problems as many T.Q.'s later than '74 need re-jetting and other mods on a non smogger application, David.