I have made grape pies for years. I find that cornstarch makes a much better thickener than flour or tapioca. The filling is very labor intensive so I do 1/2 to 1 bushel and freeze the filling for use later. One bushel of grapes makes enough filling for 40+ pies. Irene Bouchard did not invent this pie but she did popularize it. I have my mother's cookbook "The New American Cook Book" from 1941 and there is a very good recipe for grape pie (using cornstarch) in it.
This pie takes a little time to make but worth it. Served it at a dinner and everyone raved about it. Very rich so serve with vanilla ice cream!