I could be wrong here but if you are using one of those round ignition switches. It will only power the accesories hooked to to the acc. post when in the key is in the acc. position, and it will only work the other stuff hooked to the run side of the switch in the run position. But not both.
Anyway, the ford starter solenoids are way to heavy for the circuits you wish to connect. Go to your local parts supplier and ask to look through his electrical wiring/switch/relay book and you will find relays and niffty relay boxes or holders for this project. As I am not sure about the computer system you are using I can't say what amperage you need. Most of those systems use a 10 to 15 amp fuse (not much).
You said you have 12g. wire to your distributor HEI that sounds a little much but wont harm a darn thing.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"