There`s always a bunch of Luddites about. They got something running and have no idea if what they did will work on something bigger and are afraid to try. The 40" is going to be a big project for a single individual and all you need is to have to move across the country in order to keep a job and you will either sell the observatory or dismantle it and move it.
Bob Geoff is a glassworker in Tucson, Az. who is the guy that is making the HexTek mirrors (those ones that are mostly air) that have the hexagonal tubes between the front and back surfaces.
As far as a place to contact, O`Hara (and the other Jap glass places), Corning and Schott are probably the best places left to go to in order to find at least a 4" thick piece of pyrex. There just aren`t many places where you can go for stuff like this as there just isn`t that much of a need for large pieces of glass. A couple of years ago, you could have gotten a 70" piece of glass (Project 70 has the glass now and they got it from near you).
As a worst case, you may have to get the glass and mold it yourself. If you do so, you will want to do what they did for the 200" to reduce the cost of the glass (you can actually make a piece of glass cheaper with the cost of the molds included than without). Another place you might check with is Roger Angel`s Mirror Lab for a meter sized piece of glass.
Good Luck!