583 cubic incher planned.
Was just looking at James K. Wagner's Ford Trucks book and I am sure I had seen it there before, but had forgotten about it - there was also a 583(!!) cubic inch SD engine under development along with the other sizes. It never made it to production for some reason - possibly a marketing department decision or it may have had a running problem such as a vibration issue.
Since the 534 shared the bore size with the 477, but had a longer stroke crank, it's possible that the 583 would have used the 534 crank with a larger bore and if so, it suggests that a 534 block could be taken out enough for 600 cubic inches or so.
It would be cool for someone who has a 534 in a truck to take it out to 583 and bill it at shows as "The One That Could Have Been"