Do you plan to solder the sights in place too? If not, then practice cutting dovetails where they won't show, if they aren't perfect. Practice makes perfect, so practice on cutting dovetails on the underlugs so's the dovetails for the sights will look good.
Secondly, if the underlugs are soldered in place, the protruding base will need to be inlet into the bottom of the barrel channel, so there is more work for you.
Thirdly, the extra thickness of soldering the underlugs on the bottom flat will make what should be a very slim rifle look thick and clunky.
Fourth, long underlugs protruding into the ramrod channel will interfere with the ramrod, so's they will need to be reduced in length, so you will be back where you started.
IMHO, dovetail 'em in,as they should be. It will save you a lot of work and a lot of frustration, in the long run.
God bless