The T3 and Clen are going to burn calories--calories that you could use to pack on mass.

I have heard people talk about T3’s role in protein synthesis and Clen’s potential and an anti-catabolic agent (whether it is, is not conclusive by any means), so some people have worked them in to a bulking cycle, but I think it’s a mistake.

The bottom line is if you take them you will burn a lot of calories. Don’t so it for the same reason you wouldn’t run 10 miles a day while bulking: you want to use calories to build muscle in a bulking cycle.

If you are worried about gaining fat when you bulk, either monitor your food intake better (don't add too many calories) or wait to the end of the cycle / post-cycle to run the T3/Clen and burn off the fat.

I don't have any studies to back up my advice; it's pretty much off-the-cuff from my personal experience.