Question Mounting Papyrus

Mount to what?
Normal, strategically placed Washi hinges work well on papyrus. It would take a bunch, and you might refer to the Hedgehog method for this piece.
It most likely came in rolled, so it will need some time in a folder under some pressure to regain flatness, though papyrus, by nature, isn't flat.
Make sure the customer's expectations are in line with the reality of an organic handmade paper that has been painted on.
You do want to elevate the glazing so there is no contact if possible. The pigments are subject to burnishing with any contact, including fingernails.
Placing the hinges can be tricky as papyrus is translucent and most hinging materials are white and most customers want black backgrounds because that is what they see in the souvenir shops.

Best if youcan convince them to use something more neutral for a background then try colouring your hinges to a buff colour and, of course placing them behind the thicker, darker parts of the papyrus.

As WPFay said avoid glass contact as the paints used in papyrus art, particularly that "red over gold" Egyptian artists are so fond of arevery prone to flaking off or sticking to the glazing. He is also right about the cockling - a papyrus this size will never be flat and even getting tit to stop roling itself up will be a long process. My conservator treated some fairly large ones for me using a humidifying technique and that worked well. I also managed to flatten some (cheaper) ones using a steam iron very carefully through some cotton. The trick is to get the iron hot enough to create steam but not too hot or the paint will be damaged.

Good luck with this job - it sounds like a bit of a nightmare.
I like to use "hedge hog" mounting onto a white/light colour 4 or 8 ply rag board, cut about 1/4" to 1/2" smaller than the papyrus.
Attached with mulberry paper hinges and rice starch paste at several strategic places along each side of the piece.

After the art is properly attached to the mountboard, it's then very easy to attach that mountboard onto the display mat with appropriate adhesive (EVA glue, gel media, etc.) under weights.
That way it doesn't matter what colour your display mat is.
The white/light colour of the mountboard makes the translucent papyrus really bright and the colours of the art "pop" more than if just directly mounted over a neutral/dark colour mat.

I recieved 3 papyrus pieces this week from one customer.
The papyrus is different for each piece. Two are the "usual" reedy light colour that is somewhat translucent, but of different composure.
The third is a darker, almost barklike colour I have seen only a couple of times.
The two light ones are going over white rag mountboard. However, the 3rd isn't translucent, and the white would show through several holes in the dark papyrus, so it is going on a black mountboard instead.

I agree with all statements to discuss expectations with the client. Papyrus isn't flat when it is made, got "less flat" when art was applied, even "less flat" when rolled shipped and stored.
Aside from dry mounting (which is certainly not an archival process), there is no way to make papyrus stay flat while it is displayed in a frame. It is likely going to want to move and flex over time and seasons.
A properly performed "hedgehog" mount will allow that to happen without risk of harm to the papyrus.

As always... research and practice any new technique you are learning until you are proficient at it. Don't learn a new technique using a customer's artwork.

Speaking of glazing...the piece measures 24.5x73 inches.
What will the finished size of mat/glazing be?
I would strongly recommend to my clients to have acrylic instead of glass for something that size.

How much space are you providing around the perimeter of the art to the frame?
Are you float mounting or over matting?
Have you selected an appropriately deep frame?
The only thing I might add is to try Klucel-G as the adhesive. Papyrus is prone to puckering around the hinges due to its fibers expanding rather unpredictably. Klucel (being soluble in alcohol) solves the problem.
Depending on value, of course. I have used direct contact overlay with plexiglass and a good textured fabric mat as the backer. It is easy and reversible, but it depends on the paint used on the papyrus. Some of the metallic ones can be tricky.
Make sure to lower customer's expectations (and then wow them)