Does painless really not hurt?

It's a 30 or 40 hour install. If you know what you're doing.

I've built my own harnesses before, the painless unit is easier than scratch building one, but it's not a drop in.

if you get the mustang specific one it won't take that long, the wires are crimped already; that is a drop in from what i've heard.
http://www.painlessperformance.com/webcatalog/largeview.php?SearchField=20121
That's the mustang specific one right?

The
2 ea cooling fans
Holley Blue fuel pump
Remote Trunk
Alarm
Possible Electric Windows

I'm not sure how you'll be able to get all that in on the stock harness, it will take modification and research to see which wire receives current when;

Rather, i'd suggest getting a general harness; it takes a more work because it doesn't come precrimped but from the sounds of the mods that you have i think it'd be better. I used a EZ wiring kit; not painless, i used a 21 circuit, i had way too many wires left over. so i think 14 is good for your purposes. But mine had wires already labeled for electric fans, fuel pumps, trunk light/opener, electric windows, etc. I still have to call them to see how to wire an alarm i'm not sure which wire receives constant voltage.

Its not a nightmare, and its not that hard either, just take your time when replacing the wires.

I just received the 67 specific painless harness this past week. Looks really good. I kept thinking I wanted the factory replacement but after much research and advice from painless's forum I decided to get their kit. It looks really good. The factory underdash part was going to be as much cost wise as the complete painless that covers everything. Sign up for CJ Pony parts emails and they have been doing 10% off Fridays, got the entire kit for $562. Tech support said 20 hours on average to pull the old and install the new. The universal kit would take longer.

the painless or ez wire kits are not hard to install, but they are time consuming. more so the more universal the kit because you have to cut the wires to length, and terminate the ends. the nice thing though is you dont have to wire up the fuse box, have several spools of wire on hand, and work from a wiring diagram. the wires are marked every few inches as to where the wires go.

As I recall, someone on this forum was doing a thread on installing a Painless harness in their vehicle. A one time, it was supposed to be done step by step. Might want to spend a few minutes researching for the thread.