Rumor has it (and I also read it somewhere) that Dart built the rocket block for GM. The rocket block and iron eagle are identical in specs as far as I can tell. Being they are both 4.4" bore spacing blocks, with Dart saying 4.185" is the largest recommended bore for the iron eagle, I'd say don't go to 4.20". You would only have .200" between any two holes [on one side of the block (to keep the dogs at bay)].