differences are intake, heads, and cam. They have a different firing order because of the cam. yes, non HO intakes face towards the driver's side but don't let that necessarily be the deciding factor. remove the top plate on the intake. if it's an HO it'll say HO under the plate. Again though, someone could have swapped out the non HO intake for an HO intake. aside from those, they are the same motor. if you have an HO motor, you could just swap the intake, heads and cam and you'd have the same motor as any stock HO mustang.
If you are asking this question because you want to rebuild a H.O. block. And you plan on going to aftermarket heads and intake etc etc etc. There is no real need to look for a h.o. block. You can turn the non H.O. into a H.O. with a cam and all the aftermarket goodies. You may have to go with a retro style lifter to get it to be a roller or just go with a flat tappet cam with the h.o. firing order.