Just a quick input "C", based on my personal experience.
I've used M&H (
http://www.wiringharness.com/) twice (that's two different cars) for underhood harnesses. I have a hard time calling them "aftermarket", because they have the same size and color-coded wiring, and the identical connectors, so they LOOK original!!
They are definitely pricier ($$), but IMHO they are well worth it. To be able to layout and install a plug 'n' play which looks exactly like the original was great.
They can also customize ... in my case added a tach, upgraded distributor to HEI, and newer model alternator. They modified and changed the connectors to what I needed. No cutting, soldering, splicing, etc. Just lay it in, remove the old and viola it was perfect. Call and talk to them ... they are great and will work with you.
So that's my
worth,
Ogre