The block fill sounds like a good idea. Dad had one of these motors in a 56 chevy when I was young and at the strip we would come off the line at 7000RPM but that was a early model 283 .125"over with thick walls. I would be worried sick taking this motor that high if the walls come out thinner than around .150"+ with out block fill .
I havn't had it checked or sonic'd yet but it's amazeing what a difference you can fill in thickness reaching thru the water hole and pinching the cylinder walls between the thumb and index finger in comparison to a late 70's Ford 302 sitting 20 feet from it. The 283 feels alot thicker so they must have more meat than most other small blocks.