Well, Sean might blacklist me from APUG, but I think this is one of those applications where digital shines. Make a drum scan of a nice slide and have them print it on fuji crystal archive or the Kodak version.
Now a days to make an internegative will probably cost you more than a good drum scan and you cant tell if it is a good one until it is printed.

Now if you really beleive that suffering is good for the soul, then I guess I woud go to an 8x10 internegative to get really good quality on a 5'x6' mural.