Agree with the 50ish lbs. I find it helps to achaive that 50 by stepping a player into the weight to be gained.
EG: FB -> TE -> C; TE -> C -> G or T.
Never had any luck taking a te to a wr or a wr to te.
Lots of good results taking a te to a fb or rb. Especially to a RB.
Lots of good results taking a rb to a wr and vice versa. I wont use a WR shorter than 5'11".
lastly, seems every DL is actually an LB, and most LBs are better DEs in the post v4.5 to current 4.6 draft pools.
Losing weight is apparently easy. For weight gain, I suggest stepping that player into the weight gradually over a season (or 2 if going from say TE to a RT).
No one fattens a christmas hog in one summer ;)