Generally, there is more "meat" in an action made specifically for use on a rifle.
Although an action from a rifle and a shotgun may be dimensionally similar on the outside, inside there usually is more metal left in the rifle action.
Not only does this make the action inherently stronger, it also keeps a little more weight in the gun which is nicer in a rifle and helps to balance the gun a bit more since the barrels are heavier than shotgun barrels.
There was a little article in one of the recent Double Gun Journals recently on just this subject.
As for the chopper lump question, C.L. barrels seem to be preferred on a vintage rifle although there have been plenty of vintage rifles - even big bore nitro rifles made on shoe lumps too.
Monoblocking is a more "modern" practice - I am not sure when that became more prevalent as used in production gunmaking, but it is the usual practice when building up a rifle on a shotgun since most of the fitting of the lumps and other bites is already done for you. Done properly, this is a very viable method of fitting the barrels. There are many excellent examples right here on NE!
I suppose the benefit of C.L. barrels is that there is one solder joint between the breech ends of the barrels with a lot of area for a very strong joint. Each barrel being one single solid piece of steel from one end to the other. And, since they are "sandwiched" together they are inherently stronger when the gun is closed and in being fired. I say stronger, but really any accepted method of fitting barrels is quite strong and this may be splitting hairs. Chopper lumps are quite expensive though too.