I'm not an acrylic expert, but I'm familiar with the builder and have limited experience working with acrylic. I think you're going to get a significant bow at 72x27. That just too much area to keep the panel flat. I could be wrong, but I would look for a different builder if that's what they're recommending. My guess is the common consensus will be a minimum 3/4" maybe even 1".
This thread is a good place to ask about acrylic.
http://reefcentral.com/forums/showthread.php?t=1056956&page=182&highlight=acrylic