There's really 2 questions, yes? Can a Lang 36 get 70lbs of butts on there, yes, but VERY VERY tightly. Watch your air circulation carefully. Yield should be something like 45 lbs pulled.
Then, how many will this feed? If you have eaters, and all they're eating will be 1, 2, or a pig might eat 3 sandwiches, then you're looking at 3/4ths of a pound per person, so maybe 55 to 60 eaters. 70 if they're not big eaters. 80 to 100 if there's lots of other stuff for them to eat.
Ribs, another story altogether. Some just serve a few bones per person. I dont know about you, but 'round here I've average about a full rack per person, but then these folks are EATERS. Frankly, they'd probably eat 1.5 racks if I ever made that many. I've yet to have leftover ribs except for the time we cooked 16 racks and only 6 people showed up, and even then we only had a few left-over... Ribs take up a lot of room for only a small amount of meat cooked...
You might want to cook, what, 6 butts and pull them off and into a cooler (warmer) while you load-er-up and cook the ribs.
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from darn near anywhere)
Started competing in chili cookoffs back in the 1990's and have competed in more than I care to count. I became a CBJ in MiM in 2005, then MBN and in GBA in 2010. I've probably judged 130+- BBQ comps (sanctioned and unsanctioned) over this time. That said, I really enjoy competing more than I enjoy judging, and hope to get back to doing 4 or 5 a year in the near future.