When printing that large (which I often do), the first and foremost is the largest file size as possible. I always start with a 50-75 meg raw tiff file (from both my digital Canon and analog Nikon Coolscan V 35mm Slide scanner) and expand to 24"x36" at 300 DPI at 3x4 ..or much wider (3x6-3x7) for my panoramics. Thats my "working size".  If your source film is medium-large format, a high resolution drum scan is recommended. This allows mural size prints with fairly decent resolution.  Tis ram and hard drive hungry with a 256 meg -1 gig  single layer size, but I have printed archival Giclees on Silk (real Silk)  Satin Paper and Canvas up to 7 feet wide using my older Epson 10000 since 2005, and more recently with my friends newer Epson 9800. (The 10000 is still easier with ink choices though, and handles two rolls of paper at a time for easy paper roll changes)  The 10000 though did not travel with me to Seattle :( As is the size of a piano)
I have looked at some of the "enlargement" software out there, and have yet to try some yet.  I recall the early days of "Jag" lol :)
If someone has some experience with some of the newer enlargment software, I would love to hear about it. And an old school trick: if your file does not look that good enlarged, there is the use of a slight Gaussian Blur, then Unsharp Mask a few pixels above the lower Blur setting in PS. **
Oh, and far as mounting large pieces... Standard Foam Core is a No No!!! It bends, curls and gets wacky with moisture and heat changes!!!**!!!  Going with the stronger and more resiliant Gator Board, or MDF is a must, and a requirement at many galleries that have experience with large pieces. After 24+ gallery shows, I learned the hard way :)

Cheers

Kurt