Sad to say it, but pull the carbs, and replace the o rings on the manifold pipe... don't waste time trying to do it with carbs in place, I've done them many times, and you have a 20% chance of success rate and will end up pulling the carbs in the end.. trust me on that...
Also forget the RTV and any other kluge shortcuts... it doesn't work, or hold, and makes a correct repair much more time consuming when it fails...
Murphs has the orings, might as well do the thing right... make sure to completely clean all the corroded goop off the pipe, and polish it with a scotchbrite pad till is shiney also, and paint it before installing...
If the front ones aren't showing signs of leakage(which they normally don't) leave them be until they need replacing.
When installing the manifold with new seals, lightly oil them, and make sure you polished out the recesses in the head that they push into...
Pay close attention to the rings while inserting, to push evenly, so they don't roll out of position. A small flat bladed screwdriver, of a shaved popcicle stick assists for pushing a ring into place while inserting the manifold, as the rings do like to "roll" out, pushing and assiting them to get into the bores is pretty much the best advice I can give, and as I said, it can't be done without the carbs off, because you can't see the ring roll out of position... then its all start from scratch again... when its leaking.
Also, once you replace all those rings, fill the bike and burp the system before putting the carbs back on, again, trust me that if you didn't get the rings installed correctly.... you will redo it all again...
