are you using the 86 mustang motor for a 70 ford, or the 70 engine for the 86 mustang??
the 86 heads were called swirl heads, they were a one year only head that were designed to reduce emissions. That they did, and power robbed 25 horses from the 5.0 too, and are useless to build up. Additionally the 86 motors had flat top pistons with no valve reliefs, so you basically can't use any other head on that motor (there is one that will work though, twisted wedge, I think)