There is an apparent discrepancy in the lengths of the ARP and Mr. Gasket PP bolts. Page 178 of the Mr. Gasket catalog has them listed at 1 inch. Rodger reported that he measured the ARP bolts at .75 inch. Here is a length to the Mr. Gasket catalog.
http://www.mr-gasket.com/pdf/Fasteners.pdf
EDIT -- I guess Mr. Gasket could be including the size of the head in their numbers....