You're right, you don't have to change the entire harness; just splice a few wires. Here's the info on doing a factory gauge swap and a link to a site with wiring diagrams:

http://www.midnightdsigns.com/james/Installing Gauges.htm
http://hammar.dyndns.org/~djhamma/wiring/

I'm in the process of installing a 66 gauge bezel with Auto Meter gauges. I'm using a volt gauge instead of an amp gauge, which is really easy. I'm also using mechanical oil and water gauges. I plan on posting my photos and comments when I'm done. Do you have any specific questions yet?

 

Thanks I want the original look myself but my daughter is going to be driving this car so I do want gauges If I can just get her to look at them I may need to wire in a alarm lol

Thanks Jeff