IF you are doing SS and GOMAD you can definitely put on some mass. I have been doing 5/3/1 for the last 4-5 months and I have put on more mass with that than I did when I was doing SS. I think it is from the fact that the last set is always AMRAP and that you do higher rep ranges for the assistance work.
If you do 5/3/1 I would use weights that are 20% off of your current lifts. That will allow you plenty of room and allow you to hit some really good rep PR's in the last set.
What we think, or what we know, or what we believe, is in the end, of little consequence. The only thing of consequence is what we do. -John Ruskin