I can't either print or mount that big.

It's actually hard (so far impossible) to find a lab that would do the original request of 60x54 on masonite.

I was encouraged to find a lab that can do a lambda-printed fiber 60x50 print (as opposed to chromogenic RC that most everyone else is offering, never mind inkjets).

Every lab I talked to seems to be opposed to an exposed print mounted to any kind of board without framing or face mounting because the paper and the board shrink and expand differently depending on the environment. Dibond seems to be best choice for the backing (two thin aluminum plates with some foam in-between). Most labs would suggest face-mounting to plexi as well. So far the lab i liked the most (that showed me the most options and was really understanding of the aesthetics of traditional silver halide printing) is OK with exposed fiber paper but suggests thin framing to cover any shrinkages that might appear on the sides over time.

I don't know if I can convince my client to invest into a museum quality print though.