Mark VII
Reply #7 –
HO consisted of different upper/lower intake, head-('87 up used E7TE, pre '86 and earlier used E6's), different pistons, cam, injectors, computer, and I think the compression was a bit higher as well. And the firing order is same as a 351W.
I'm sure there are a few other differences between the SO and the HO that I forgot.
Doesn't take a whole lot to switch from SO to HO, either.