not sure on the temp danger zone, but I've smoked a quarted hog before, the quarters weighed 18-20 lb and it worked out good...took a while though. When they get that big the 90min per lb logic goes out the window...it will not take you near 30 hours to do it...But it might take 20+.
Just a note: Mine still had the hide on it...I made checkerboard scores through the hide and smoked it with the hide on...it looked like a pineapple when those cuts opened up and wow did a lot of fat render out...made a huge mess. If I had to do again I'd remove the hide and alot of the under-fat...pork shoulder has plenty of fat inside...you'll never dry it out...It will be a lot less greasy that way too. Besides with the hide on you cant get bark