They can be all the same size, as on my 1981 and 1984 258ci engines. They have been changed over the years and my manual shows the different size lines.
The marks on the block are FROM THE RIGHT +2deg, 0, -2, -4, -6, -8 etc.
What you set the ignition to and at what engine speed depends on what system you have. If you have the computer it will be much more advanced than a non-computer setup. If the computer, which adjusts your timing for emisisons etc, has been removed your distributor will have limited advance capabilities.
The carburettor choice should not, IMHO, change the ignition timing but like all things Jeep the advance may not work as intended and you may get pinging anyway. I manually rotate the distributor until the pinging just stops under load, this is the ideal setting for that engine. It would obviously be better to sort out the distributor advance / vaccuum porting than the hit or miss approach but it works.
The timing marks allow a good initial setup but to do that you need to see the amount of advance you are setting. If you do not have timing marks you have to find TDC and work back.
I use a Gunson Xenon timing light that allows me to use a dial on the back to alter (advance or retard)the flashes. This appears to make the TDC 0 lines line up and then I can read the actual advance off the back of the gun. This avoids the need to know anything apart from TDC and very useful / pointless depending on your view of such gadgets.