A question here, how do you intend to enlarge your ULF negative into an 80" wide print? Do you have an ULF enlarger. Have you explored just "WHO" might be able to do this. There probably aren't any labs anymore that can do this. Your alternative is a drum-scan and digital print.
In practical terms, an 8x10 Portra negative will be so grain free and so sharp (depending on the quality of your lens) at the size you want to go, that I doubt you would want anything larger, and finding a lab that can handle 8x10, while rare, is not impossible, if you search on a worldwide basis.
the "primary" use for ULF is to make killer contact prints, the same size as the negative.