Flathead Coolant Restrictor Hole Size

I'm installing coolant flow restrictors on my flathead instead of thermostats. What size hole should I use? I've heard 1/4" up to 5/8"?!

 

why are you switching?....i've always heard 5/8 though

 

Yes, last year I ran into a problem where my original type thermostat stuck on one side and this engine overheated so fast I could't believe it. Actually burnt the paint right off the head. I think there is just too much crud in the passages of an untanked flathead.I removed both of them and installed flatwashers with a 5/8 hole. My washers were just large enough to stay in place in the hoses but if you want, you can tack weld them right onto the head outlet to hold them in place. I haven't had any heat problems since. Runs about 170-185 average on a 80 degree day. Never overheats in traffic, probably due to an electric fan.

 

I have Edelbrock heads on my 59A and the stock thermostats will not fit in the waternecks. I ran all last summer w/o restrictors or thermostats, however she got pretty hot during the heat of the day (90+ degrees). It never boiled over, though I dumped about a quart of water each day (unpressurized system). I welded some fender washers onto my coolant runners (chrome $11 exhaust tips) and installed them last night (w/ 5/8" holes). I just wanted to know if I needed to reduce the restrictor hole size.