My bet's on the ignition switch or wiring harness.... or both. The coil wouldn't be an issue....
The only other thing would be proper voltage getting sucked by something. Bad connection some place, on the sylenoid or a relay some place... Did you replace the tin box or the aluminum one? the aluminum one is the one that they will have in stock most places.... BigJim would know more about this, it's found also on a lot of fords. the tin box is different, located across the engine from the aluminum one and has some wires that meet up with the alternator and some other things, it is the relay box for lots of stuff, located in a high place near the grill so it will stay dry... yea, right....

Okay, have a nice day and good luck! let us know what you find out so we can learn from this.