i know it sounds odd and im no fool i know what a 283 looks like and i know what 4 bolt main looks like i have worked on both 2 and 4 bolt as for the block it is a 283 with a bore of 3.97 i ran the casting number and it is a 283 i still dont see why anyonw would do that though would it make it able to rev higher? that was my evaluation of why smoeone would have done it