Agree with Guerry. Not sure it matters, but why are you going to cure a shoulder for a ham instead of curing a ham for a ham?
If you're cooking a butt as a ham, I'm honestly not sure what to tell you as far as cooking time and temps.
A ham doesn't have the fat marbling or as much connective tissue as a butt (I don't think) and may be pretty tough to eat if only cooked to 155-160 (the typical ham finished temp).
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team