A couple hundred years ago (or at least that's the way it seems) I used to use the washers in my flatties. My recollection was that 5/8" was the best all around size for them = and I think that they were standard 9/16" hardware washers (they have ~a 5/8" hole). A low restriction thermostat with a couple of extra 3/16" holes today works now at least for me.

Here's where my water pump came from as well as the 'stat:http://www.stewartcomponents.com/tec...ech_Tips_3.htm

Some good background info on cooling as well (my pump is the Stage 1 #16113)