If you have a broken carb body casting, it is probably best the replace the casting. Two reasonable approaches are:
1) find a carb body or carb bank that has the same set up stamp on them and swap out either that carb position or the whole bank.
I suspect your carbs have the 627B set up stamp found on the top right side carb mount flange. If so, these are the ones to shop for. Mistrust sellers that would try to convince you they are all the same, or are unable to tell you what set up stamp is on the carbs.
2) Strip the broken carb position from your 550 bank (it is probably 022A set up). Dismantle the carb, and move the slow jet, main jet, emulsion tube behind the main jet, and slide needle from your 627B carb to your 022A carb and reassemble into the 627B bank. I'm fairly certain these parts make the setups unique and transferring the internal parts should also transfer the set up configuration. Use the slide needle position as found on the 627B carbs. I might even swap the air bleed screw, particularly if I could find any difference between them. But, I think only the F model 069A carbs had different ones.
During the transfer it would be best to renew ALL the o rings and seals in the entire carb bank. You will most certainly need to synchronize the carbs after reassembly.
I ran a 74 Cb550 with a 4 into 2 for over 10 years and no carb changes. Still used the stock airbox, but with a UNI foam filter installed. Very streetable and made pretty good power on the top end, too. I finally put the 4 into 4's back on it, mostly for nostalgia reasons. But, I didn't notice a lot of performance difference. Certainly runs the way it should.
Cheers,
