T valve or banderlog valve or manual gate valve up by the seat. If you run them dry you can get a ticket and blue the crap out of them. Or get them coated or buy new coated headers.

A gate valve off of the block isn't a bad idea for an emergency back up. I have one setup like that on my hondo before my t valve. The point of the t valve or banderlog or manual control is to control water flow into headers at different running speeds. At idle there should barely be water going into the headers. At faster speeds the increase in pump pressure opens the valve and dumps more water cooling the headers a little and dampening the noise a little. A gate valve just straight off the block wouldn't give you the volume you'd want when you're up to speed.