erratic timing on 360

If the car has a chain tensioner there should be no slop in the chain
the tensioners work so please no tensioner bs
remove the plugs
rotate engine to tdc - line up the marks
cap off distributor
with breaker bar rotate the engine backwards [ccw]
Distributor should turn, see how far the breaker bar moves before it turns, it should turn right away...if it does not time to disassemble and do the work properly.
If a shop rebuilt this engine and did not have the rotating assy balanced it is time for a new shop. Stock 360's had poor engine balance esp in the late 70's