A 2.25 pipe would be easier than 2.5 for the average muffler shop to bend without kinking, especially behind the muffler and up over the rear axle. Matter of fact, I've had trouble getting even a 2.25 inch exhaust system made by a muffler shop for a slant six, even for the front section coming off the exhaust manifold. A really good muffler shop could do it without any problem, but you might have to shop around a bit to find one.
I went with 2.25 inch Accurate LTD for my slant six because I could put it in myself, and it fit perfectly. On sale, the cost wasn't so bad. Not sure what muffler shops charge these days as I haven't used one in several decades, but I think that anymore they aren't all that cheap either.
