You've gotten good info here, and your plan is not a bad one. But, if you want to be seen by each of the 8 Ivys, I would do Stanford (either July or August session) and HF (does not matter Sac or Long Island). Unless things changed just this past year, there are 2 or 3 Ivys that do not go to HF and there are 2 that I know that do not do Stanford. But, if you do both, you will have a chance to be seen by all 8. My Ivy bound 2016 RHP did the following and it worked out well for him:
Summer before Sophomore year - HF Long Island (just to get his name out there and to get acclimated to the scene/experience)
Summer before Junior year - HF Long Island and Stanford All-Star II Camp in August (one was the week after the other).
If I had to pick one over the other, I agree, it would be HF. The main reason is that at Stanford games are spread out over 3 locations. Also, at HF, they strongly encourage and even will facilitate direct dialogue with coaches. But, we also would do Stanford all over again, if I had a 2nd son.
Good luck. PM me with any specific questions.