The difference between plate glass and pyrex is often blown up out of porportion to what it really is. Pyrex does change size with temp a lot less than plate glass does but considering the use, using a thin piece of plate may actually end up with a better mirror than a thick pyrex mirror. I believe that you have found Dan Cassaro and his glass mine of 7/8" plate. The source is a good one and you may want to do a 8" for a start and also get a 16" at the same time you buy the 8" as he's going to be running out of glass at some time and it would be a sad thing to finish up the 8" and then find out that he's out of glass for a 16" piece. If you decide that the 16" isn't for you, you can always get your money out later by selling it.
Get the glass and have fun. Remember that the 100" scope at Mt. Wilson is made from old wine bottles!