Take a good look at bibby's post, dude.
His
FIRST suggestion (and the cheapest charging system enhancement you can do) was to upgrade your vehicle's "Big 3"
The thread detailing this process can
be found here.
The suggestions following that were to upgrade the battery and if you still have problems (after following the previous two suggestions)
then make a move to a high output alternator.
In response you ask him what his suggestion for the "best" cap for your setup might be.
Nowhere in his post did he mention the use of a stiffening capacitor. If you choose to be obstinate on purpose then here's an answer for you:
Go get butt-***** by buying a 1.5~2 farad capacitor. Make certain that you follow the suggestion of whatever retailer you acquire the cap from in regards to buying a wiring kit (as I'm certain they'll try to suggestive-sell you one).
After spending that $300 give or take and it doesn't provide the results you're after please do NOT come back here whining about it.
We won't care at that point.
Caps
do have their uses to a minimal degree. The problem you're trying to cure won't be solved with one and we're trying to steer you the right way but for some reason unbeknownst to the rest of us you adamantly refuse to listen.
Have a nice day.
Homie.