340 with a 273 distributor

Simple. Use your timing light. I "personally" distrust "dial up" lights (knob which is supposed to be accurate as to degrees) and tend to use older, simpler lights, with the balancer having been marked. This is easy. Take a small thin tape and measure around the balancer, then figure how many degrees per inch and use anything you like including masking tap, to mark the balancer out to 40 degrees.

Bear in mind that "the book" specs are in DISTRIBUTOR degrees which are half as much as crank degrees, same with the specs on RPM.