Also make sure you can get it out of the door of your shop.
I did one this large about 5 years ago. What a pain.
Is the poster of any value?
Mine was an old advertising poster that the client paid $12,000 for.
We built strainer, backing, poster, acrylic package and sealed it with heavy duty clear packing tape. The 4 rails were designed so that we could bolt them together at the clients home.
The whole thing weighed about 200 lb. and was hung with z-bar. It still looks great and has held up well over the years.
The client has had the room painted and the painters moved this monster with no problems.
We can get it out the doors, that not a problem. We can also get it into his home. This poster is a B/W graphic from an Abercrombie and Fitch store display. I think I going to mount it on a joined Luann panel with paste. It get a large black frame.
If you are going to use the 1/4" Luann "door skin" panels, you'll need to build a fairly complex cradling system for the back. The Luann tends to warp out with the smallest amount of humidity. Were you planning on sealing and filling the panels before mounting so the grain doesn't show?
Another option would be to have one of the poster conservation services mount the poster to a linen/cotton fabric, and stretch it over a frame. I don't know if this is bigger than their capability, but I have seen some monster posters mounted this way.