[Okiesnake]

It really doesn’t matter where you mount the valve (going to or from the heater box). You only need one valve. Vintage Air offers 2 cable heater control valves. 46104-VUH (pull to close) for $18.50 and 46105-VUH (pull to open) for $18.00.
Vintage Air has a picture of the part as well as a price sheet catalog on their web site. Their phone number is 1-800-862-6658
There is an arrow on the valve as to the direction of water flow the valve is mounted. If mounted incorrectly it may not shut off completely, wear out prematurely or creep partially open. You don't have to mount a cable to remotely activate the valve. You could open the hood and manually turn it by hand. The old MG's had a "winter/summer" mode under the hood.