The plate is 6065 t651 Aircraft quality alum. and all of the bolts are 1/2 coarse threaded bolts. I went with the 1/2" thick so that the input shaft and the pilot bearing have the proper ingaugement, if you use anything thicker you will not be into the pilot bearing far enough. I decided that 1/2 was thicker than the bell housing, so the bellhousing would most likely fail before the adapter plate.
For those interested here is a few more pics.
Dieseltim
