They are locking devices for the mount bolts. The torque on the exhaust bolts are pretty low (I think 20ft lbs.)
FYI - You can get 2 1/2" manifolds for your 283 a Orieley's for around $70 each. They are made in China, but seem to be a good piece to use to open up the exhaust. They are modeled after the early 60's corvette manifold.
