i have a dpms lo pro classic upper with the 16" bull barrel.  i would like to install a set of yhm freefloat handguards on it.  my question is....  i want to install a longer set of handguards on it so the barrel is more covered.  the gas block ( small/square with pit rail) has 2 set screws and is easily removed....can i just buy the rifle length handguards and just install a rifle lenght gas tube?   or is it a lot more work?   thanks in advance

I dont know if the rail will go over the gasblock... you may have to get a low-profile gasblock but yes basically thats the extent of the install

edit I lied I misread the part where ou wanted to make it rifle length... that would require filling in the gas port and drilling a new one... I dont think rifle length gas systems work well on 16in barrels... but you can keep the carbine gas system and use a low pro gas block and go with a longer rail system that is free floated

Putting a YHM free float over that gas block will be a pretty tight fit. Mine just barely clears. If you were to clearance the bottom edges of the block it would fit better.

I have a lo pro classic that came with a free float tube.  It uses the same small gas block as your's I believe.  My tube will unscrew over the gas block.  I think you will be fine if you buy a midlength or rifle length tube.