The 283 probably has a set of mounts that bolt to the front of the block beside the harmonic balancer, right? The 350 block has the same bolt holes drilled/tapped, so your 65 mounts will bolt right on. Just remove the side mounts on the 350, and re-use the 65 mounting system.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!