I don't really think of 30x40" as extremely large. It's a big print, but not out of the range of what many people typically do for exhibition from negs anywhere from medium format to 8x10". If you're going to send it out to a lab anyway for a LightJet or other digital output, why not just use a lab to make conventional prints? You can always ask for proofs and work out the dodging/burning from the proofs or print a master print to your own specifications in smaller format and ask the lab to match it in a larger size.

If you want to go with digital output, look into the services offered by West Coast Imaging. They've replaced LightJet with Chromira printing for color, which produces cleaner colors than LightJet on Fuji Crystal Archive (I've ordered prints from the same scan on both), and they also offer Ultrachrome and Piezography prints. I suspect Piezography is your best bet for digital B&W.