I think the Pertronix or one like it is probably the easiest. Basically you use the stock distributor and replace the points with the Pertronix unit. This is what I have in my 74 trail rig and I am very happy with it. You can keep your old points around in case the Pertronix dies on you for some reason. You can use your old coil, but I would recommend upgrading it.
Like Steve said the Duraspark is a good system and the parts are cheap and plentiful. More to hooking it up though but not too bad.
My 76 has the Mallory MSD ignition box, Mallory coil and Mallory distributor. This is the most expensive of the 3 I listed..
The easiest is the Pertronix and the cheapest is the Duraspark, if you get the parts from a junkyard otherwise it's about the same as the Pertronix.
Good luck
Greg