We could argue these points all day and into the night I guess.
However these mods are for engines that do higher rpm than our engines turn.
The reason to have a bigger diameter cap is to stop the spark from jumping to the last or next wire in the dist cap at high rpm.
If your engine isn't missing at the highest rpm you use there will be NO GAIN from putting foreign parts in the ignition system.
Sure these parts will work the same as the ones you have now at the rpm you use...but as for a possible GAIN?
All there will be is that famous LIGHTER POCKETBOOK!
There will be no fuel savings, no more horsepower, no smoother idle, in fact no more anything!.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />
From your profile, I'm guessing you are asking about the '79 CJ. You can upgrade the ignition for more reliability. The upgrade is called the "TFI" upgrade. Basically, with the 258 that has the Motorcraft ignition, you replace the cap, rotor, cap adapter, coil and wire set with parts from a later model Ford 300 straight six. (AMC V8's use parts from a late model Ford 351 or 460 V8.) The parts for the upgrade cost about the same as the stock parts, and are usually easier to find at a parts store. The upgrade allows you to run a wider gap on the spark plugs which may make a bit more power, or at least make it run more smoothly. The later style cap connections are more secure than the old original type that came on the early Duraspark ignition system.
Here's one excellent write up:
http://www.jeepz.com/forum/engine/16904-tfi-ignition-upgrade.htmlAnd another excellent link:
http://www.geocities.com/cantwait_forviolence/cj_tfi.html 
