I saw a Galen Rowell print that had to be 2 feet by 3 feet. It was made from a 35mm negative. I have to admit, it did not appeal to me but, the man made a decent living shooting 35mm and enlarging to sizes that most would consider "not doable form 35mm".
I say try it and see...but if you are not comfortable with the result, do not show it to the potential customer. Only show work that you feel good about. Only show your best. I'd also try 11x14 as a possible compromise (it is also closer to full frame for 35mm).