From the info, this is really a WAG. I think you have two separate problems. A horn ground at the steering wheel that should be "open" until pressed, and (a real guess here) a jumper from the solenoid terminal to the starter main lead, and you've either neglected to put in the necessary "Ford" style solenoid, or you've installed it either wrongly, or the solenoid is defective. Is there a chance in he11 that I'm right on either account? As noted, purely guesses.