Charlie,
You said that you were stack cutting, how are you holding the two pieces together? Using double sided tape or some other method? I was thinking that getting the two apart when they are that large could be a challange.
Also, what is the plan for mounting? It seems to me that you might need a thicker backing for it to hold it flat, 66" of 1/4" plywood is sure to warp.
I have had trouble with both, stack cutting 1/4" plywood using the cheapest of carpet tape and trying to get them apart causing breakage, and even when the pattern is only 12" wide, I have had warping by not using anything but a frame to mount the project.
Wish you the best of luck and, I too am watching to see how it goes. I would like to try something larger (not as large as yours but...) a challange is a challange. LOL
I have never tried the sprial blades, (I was warned not to by another scroller), but I just might get some and see what I can do with them.
(He said that they streach and are hard to keep following a line.)
Rog