The aspect ratio shouldn't be any issue at all. The only time that really matters is if it becomes a problem for the stock that they normally use. A 38" print might well be an issue if their stock comes in 37". In any case you'd just lock the proportions and size it down to a size that works and call it a day. If your 12x36 print actually has to be 11.5x36, who really cares. Now, if you're wanting to use off the shelf frame/matte, then that would matter. Not the print, though.