Yeah, a chunk of pork like that is going to go 8-10 hrs or more at that temp. Glad to hear you had decent control over the heat. That's half the battle right there. The 300 near the firebox would've sped up your cook but then you have to worry about radiant heat overdoing the outside of the meat. You can slide it over there if you wrap it in foil and turn it once in a while. (I put a piece of foil under the wrapped meat so it doesn't stick to the grate and tear)
BTW, you can do pork in 6 hrs but you have to jack up the heat. When I do it hot n fast, I inject with a combination of apple juice, worcestershire, spices etc. (look up Bob Lilly's injection), rub and then smoke at 300 or so. Once the internal temp gets to 165 or so, I wrap it in foil and let it go until it's tender (internal 195-200ish). The finished product is every bit as good as doing it low n slow. Pork is very forgiving.