the difference in blocks is that the d16 is a stroked out d15, so basically just the crank/rod legnth, the rods of the d16a6 should also be stronger then the d15's. all pistons in d series engines will interchange, but the compression ratio will change.
this may help:
http://nicotine.golden.net/D-Series/dseries.xls
wall thickness should be exactly the same.