I've used either OMC gasket sealer or Permatex Aviation. When you get the bellows on the gimble housing end, make sure that the rib in the bellows sealing surface, fits into the groove in the gimble bearing mount.
Also, the way I've done it is to disconnect the transom shift cable up at the engine bracket (just pull out the cotter pin, don't change the adjustment at the bellcrank) to allow the pivot housing to move back a bit (without disconnecting the cable at that end), it is in the end easier to pull back the pivot housing than to try to remove the exhaust bellows first and then jam the new drive shaft bellows in above where the exhaust bellows was with the pivot housing in place......screw fitting on bellows clamp should be between 1:00-->2:00 o'clock position....if you do remove the pivot housing, there are 2 fiber washers on each side, that may fall down...keep an eye out for them...I replaced them the last time I did the bellows back in '16...