Usually if the block size is wrong, the magic number for the configuration block is also wrong (since the configuration block is then looked for at the wrong offset), which should cause a panic or other drastic failure.
dat.h looks pretty much the same in the 64-bit fs. The block size is a truly fundamental constant of the file server implementation; changing it currently requires recompilation because the block size, and constants derived from it, are used in array declarations and the like. There are ways that one could work around it, with some contortions in the code. kfs does, after all, but kfs had to (or at least did) change the on-disk representation to make this feasible.
