The firing order is the difference. It's suppossed to make more power and run better. The camshaft is what determines whether it's an HO firing order or not. Then it's a matter of getting your wires right. There may be more but thats all I know.
I think its the cam and heads. The heads in your '85 are E5s (I think) as opposed to the E7s found in 87-93s but compared to other cars with 5.0s in them of the time (crown vics and such) I believe the Mustang's stock cam is more aggressive and the only other vehicles the heads are found on is trucks. I might be wrong though so somebody else chime in.
as far as 87-93 goes, HO is determined by camshaft and computer, which determine the firing order, and possibly wiring harness. forged pistons, roller cam, and all that are just nice enhancements only done to the HO, but they dont MAKE the engine an HO
you know that the newest chevy 350's finally are using the same firing order ford uses with the 302 H.O.'s. if you were to number chevy cylinders the same way ford does the new vette engines would have the same firing order. it is the most efficient firing order makes most power due to the cooling time inbetween side by side cylinder firing. if that makes any sense. wtg ford and finally chevy.
My coupe has an (aftermarket) 302 non-HO cam and it makes more power then a stock 5.0 HO with a 351 fireing order. i never understood the logic behind this (car came with it) but for some reason this is still a perfomance cam, working with factory EEC...