Yeah, if the blocks are placed in order over time that could reduce the strain on the system. My laptop can't even fill in a 50^3 area sometimes. If you're going to auto-fill that build helper area don't do it all at once.
You know I'd also like a flood-fill macro that would place the selected block in empty spaces adjacent to the one you're looking at, then spaces adjacent to those, so on and so forth until A) there are no empty spaces adjacent to blocks placed in the last step or B) the game recognizes it's trying to do too many actions at the same time (a server config option to tell it "stop fill-in if number of blocks in next step exceeds y" - so a couple players using the fill-in function on a server doesn't crash it) or C) the player hits a key that ends the fill-in setup, or D) the next block placed would extend the dimensions of the ship in any direction beyond what they currently are (in case it tries to fill in void space outside your hull because you had a leak, this will tell it to stop)
Course that goes beyond the suggestion of filling in your build helper area.