From all the varying accounts of tightness and/or normalcy with 8UJ I'm convinced that each pair is different. The 26W Matics that I sold measured about 14.25" flat, but I saw another 26W on eBay that measured an inch more than that.
The cut is meant to be stacked. Far as fit, I think these will stretch - I got them in my 71J size and the waist is a smidge on the tight side. Of course, yours could be strangling you. Dumb thought but maybe, just maybe, you should just seek out another 31W that measures on the larger side... if you catch my drift.