Best Of

keep it open but also do what’s more convenient. This is pretty insignificant

@clammy 's point is safety critical, you need to one way or another arrange it so there is a relief valve connected to the boiler with no valves between it and the boiler, that is what code requires for the relief valve.

Edit:
especially if you're going to run the gas valve bypassed without the safeties.

That is a older prv/ safety the valve after the valve after should not be closed due to closing it in effect removes the safety valve from doing it job in relieving the boiler should it s pressure become over 30 psi . Those type of feed safety are available but I believe against code due to usually have a shut after making it against code . If the proper prv/ backflow is installed then a tee should be installed and a 30 psi safety valve should be installed . If this was indeed a gravity system and not converted don’t assume that all the radiator where full of water in some cases the upper floor radiators where not completely bleed leaving a little bit of cushion for expansion . As other may have stated the existing expansion does not have the usually tank style drain w dip tube at the age of your system I would wish you luck in removing the plug without damaging its thread in which case you will need a new expansion tank being most gravity system when a bladder type tank is installed will need some type of air seperators to remove the dissolved o2 in the feed water which means possible so repiping .It’s great to keep a antique going but as for eff I would image it’s quite low and that if removed and a new modern cast iron hot water boiler where to be installed and properly piped as a conversion from gravity to pumped system not only would the time to come to temp and fuel comsumpition would surely be dramaticly lower . Not to beat a dead horse there’s a lot more to a proper gravity to pumped conversion then just some piping if you would like your new modern boiler to have a long non condensing lifespan . It s more then cutting reducing the piping there flushing the decades of rust which always form in gravity hot water systems and installing mag seperators and possibly strainers to further remove . Your bigger expense will be removal the asbestos the boiler breaking and removal of the boiler will be far less costly then the asbestos removal . In the past I ve done a few of these and the results and speed of which the system heated where drastic and most had a mim of a 40% fuel reduction . Key is balancing the two mains ,staying above flue gas condensation and keeping the velocity fairly low in the piping .
peace and good luck clammy