A couple of things- Cen-Tech 40963, Harbor Freight $29.99, adjustable knob on end. I don't believe it will work with a MSD ignition box.To time using an MSD ignition box use MSD 8990 timing light, which isn't the knob adjustable one. 

Advance......Auto zoo.....shoot Walmart might even.
Don't fool with the 'dial-back...smile-back' ones.....you don't need it .
Set initial timing about 12-14 BTDC. Verify TDC on the engine...buy a timing tape and put it on the damper.
That way when the timing light hits it you can 'see' what the timing is at different RPM's.
6sal6

Biggest question is whether or not you want the dial back feature or not.  If you have a timing tape installed or a balancer with marks out to say 50 degrees BTDC you don''t need it.  If you only basically have a TDC mark you have to decide would you rather spend a little more on a light to get the dial back feature or install a timing tape. 

While they may cost more, consider one with an RPM readout.  Helps with both initial and full advance timing settings, as well as those in between.