Major difference is firing order. non-HO is 1-5-4... HO is 1-3-7...
Some non-HO motors have roller cams esp. the late model Crown Vics. Most have flat top pistons which limits your cam lift to basically stock
If you have an Non-HO motor in an HO Mustang, you need to swap the camshaft to an HO cam or it won't run right
Camshaft is the main difference.....that's what changes the firing order. The motor doesn't have to be roller since the 84 H.O. wasn't roller and it didn't have forged pistons either......I believe it's just the camshaft.