318 with 340x heads sluggish

Could be a combo. Without knowing the cam or where it is installed (cam timing, "degree") that could be a huge issue as well
ANY used engine........without hard and true documentation......is a crap shoot.

Also, how do you know that the timing is "where it ought'a be" and chances are "it ain't."

Timing, distributor curve, cam specs, cam timing, and even the carb, and absolutely the intake manifold??

What does it have for exhaust manifolds / headers?

A lower RPM stall converter is the LAST thing you want for an engine sluggish on the bottom end, or did you mis--type that?