I have a 1973 302, automatic, tilt wheel mustang. Car will turn over but not crank without jumping the coil direct from battery. once started it may die without hesitation. It is getting worse, the last time I drove it I had to leave the coil hooked directly to the battery for it to stay running. I have replaced the coil and checked connections. I am figuring it is in the ignition harness ( I have replaced the ignition key cylinder) or the Mallory electronic distributor. I have purchased the ignition harness, question is, how do you get to it in the steering column, and second, any other advise or ideas of where the problem could be?  

Slow down a minute!  The switch is at the bottom of the column and you don't have to disassemble much to get to it.

What do you mean the car turns over but won't crank? (this sounds contradictory to me) Do you mean it cranks but won't start unless you jumper 12V to the coil?

Have you taken a wiring diagram and a multimeter and looked for why you don't have power to the coil? (or if that's even the problem)
To trouble-shoot your problem, place the ign switch to run and see if theres voltage to the coil, it will probably be 8-5 to 10V because of a inline resistor. if not voltage go to the easiest spot to get at and check for voltage out of the ign switch. If it's got power there it's in the wiring in between.
You mention you have a mallory elect ign, whats is the hook-up difference between that and the stock ign system? (you need to look there too)  

Changed the ignition with no problem and car fired right up 3 times, then it would not on 4th try, motor turns over but will not fire. Test light is bright on the battery but dim on the coil, so I know voltage is being decreased somewhere in the wiring.
Mallory ignition was on the car when I bought it. It has an electronic eye inside the distributor. One wire runs from the coil and goes to the firewall and is connected to a ceramic reducer then contiues from the reducer to the distributor. The ceramic reducer is a Mopar part, I do not know the reason for doing this, but it is how I bought it. I have had no trouble starting the car in 4 years until now. This is the only difference from stck that I am aware of.