One side of the cap should have a stripe running down it, with - - - inside running in the same direction- that will be on the same side as the negative lead:
The shorter leg is usually the negative, so you should be fine.
the short side is negative....the white strip on the side of the capacitor should confirm this.
I run my MM with a HUGE cap on a 1515 1.5d on 4s in a 12lb maxx. I put a fan on it only after i knew it could run without one. Otherwise the fan is just covering up a "symptom"