My guess is to keep compression at a manageable level, (317's have large chambers) but I will let Cline speak for himself.
Most likely, but compression could also have been handled with piston selection since he is replacing them too. That's why I hope he chimes in. It clearly was planned out and not cobbled together. I'm not familiar with all of the intricacies of Gen III/IV builds, so any info I can gain will only make me better.