The engine is shutdown by applying a vacuum to the shutdown valve on the injector pump. A switchover valve is a three position vacuum solenoid where one port goes to atmosphere, one to vacuum and one to the IP. When off, the IP sees atmosphere (no vacuum) and when activated sees vacuum.
You could use a vacuum solenoid (as I did initially on my swap) but you'd need to put a pinhole in the line to bleed off vacuum when power is off (so you can start the engine). I'm currently using a Mazda EGR purge valve for mine. If you source it at the junk yard, grab the electrical pigtail for ease of installation.
You can modify your glow relay to use the 80amp fuse and the glow harness if you want to work well with the Ford solenoid. (Details here -
http://www.peachparts.com/shopforum/2985078-post8.html) Some people have had problems with the solenoids, but I've had mine (pulled at the junk yard) in for 108k miles without a hiccup.