Here are the differences that i know of:
the HO has:
a double roller timing chain stock, non-HO's got the silent type
351W based camshaft (on roller cams, i don't know if it's still the 351W camshaft anymore, but it retains the 351 firing order)
Reverse rotation water pump for the serpentine belt (i think the non-HO's got this in the mid-late 80's compared to the HO's getting it in '82)
HO's had forged aluminum pistsons on most years, whereas non-HO's always had cast.
And in general the HO's always had a better induction system than the non-HO's and some years had much better exhaust.
As far as rebuilding an engine is concerned, you shouldn't even worry whether it's a HO or non-HO block, the only difference lies in when you get into roller cam and non-roller cam. The roller cam blocks have higher lifter bores to retain the roller lifter better. But, as previously mentioned, you can buy kits to convert a non roller cam block to a roller camshaft.
Good luck!
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT