I don't know the measurements of the 5.8, but you might also want to consider the 580 main sail since it is about 4 inches taller than the f18 and is set up to work without a boom. Actually for the 580 you can get it either with or without a boom, so be specific when you order it. With the f18 sail you probably would want to put a boom on it.
Trey, Just fess up and tell us what you want to do......I looked thru all my old (before your were born or playing with GI Joe at least) Nacra class including the N-5.8 rules I dug out of the archives and I guess you have to call Skip as He made them all and there was no measurements in there....There are lots of sails that would probably fit with minor modification.