well that depends, is the car stock? does it have a bucnh of electrical add-ons like electric fan, power windows, etc?
if it is mostly stock electrical stuff and no extra power goodies the a stock harness would be fine. if you want a bunh of extra stuff like power windows, locks, seats, mirrors and stuff like that a painless type harness might be the better choice.
I did the regular harness when I first fired mine up and it has been fine, planning on going electric on fan and water pump soon so I will probably just modify mine.
The painless is pretty expensive but I can't comment on it.
It seems like Virginia Classic Mustang had a price $100 or so cheaper than alot of vendors.<a href="http://www.smileycentral.com/?partner=ZSzeb008_ZNxdm117AAUS" target="_blank"><img src="http://smileys.smileycentral.com/cat/36/36_11_23.gif" alt="Electric" border="0"></a>