Ray,
The plumbing thing aint that hard, judging from all the other things have got done thus far it should be a cake walk for you. I just did mine for my 460 a couple weeks ago, just keep in mind, it is a much more simple design than you would think.
In answer to your question though, YES run a ball valve on your overboard dump line. Reason being: if you 'choke' your water inlet line to your motor to build temp (pretty normal thing to do right?) and you dont have a ball valve on your dump line your headers will starve for water because the now lower amount of water running through your motor is going out your dumpline....SO, if had a ball valve on your dump line you could 'choke' that and put more water back into your headers.
Does that even make sense? I swear it does to me. Maybe I'm losing it....either way, you'll get it, but the ball valve on the dump line is a must. My $.02 is in now...
Nick