The three bolts that secure the engine cradle to the engine (back side of the engine) are two 3/8-24 x 4, 1/4" and one 5/16-24 x 4, 1/4". If you choose to use thick washers (two per fastener) you can use 4, 1/2" lengths. CNW has a very sexy set of ARP fasteners that you can simply install and go worry about something else, of which, with a Norton, there are plenty of options.
However, it seems that you solved your problem, so this is for individuals that have yet to embark on replacing the fasteners in question.