The difference is ported vacuum is above the throttle plates, is zero at idle, will increase as the throttle is opened (to a max of about 12inHg) and will slowly return to near zero when wide open throttle.
Manifold is below the throttle plate, is a max at idle and will drop with the throttle opening. For 99% of vehicles, ported is prefered but feel free to experiment.
Start the timing at about 8-10* BTDC. Engine vacuum can vary from person to person, but at idle should pull about 20 inHg (steady reading). Since it is Nuttered, once the vacuum advance hose is removed from the distributor, it won't make much difference in RPM's as the mechanical advance won't kick in until about 1600 RPM's. You can also time it using engine vacuum, adjust dizzy until highest vacuum reading is obtained, then back off 1-2 inches. Mine pings when I go that route but yours might be different.
1986 CJ7, 4.2 w/4.0 head, TFI-HEI hybrid ignition, Clifford manifold w/Holley 390 w/cold air intake, OBA, 4.5" lift, Woody CV shaft and Tattons in front, 4.10 gears - lunchbox in front, Truetrac in the back, twin-sticked, blower upgrade for running topless, trying to keep it simple.
It's just a Jeep, and if you don't wheel it once in a while, it's not even that.