my used crate motor im rebuiding doesnt have a damper on it. What size would it be. Its a Targetmaster 350. Also, would a stock damper be fine for around 400 hp?

Blaster a stock damper 6-3/4" or 8" will do.I prefer a 8" with big power small blocks, some guys might chime in to differ. I believe they still cost around $60 for a stock damper,and I ran many SB over 400 hp.Good luck!

Blaster,
Just make certain that whatever damper you get will work with the timing tab supplied with the engine. On your type of engine, any stock damper should be just fine.