I have a 1966 289 Mustang Coupe.  It currently has the C4 auto.  I want to swap it out for a World Class T-5.  How big of a deal is it really.  Has anyone here done this before?  From what i understand the biggest part is fabricating the pedal setup, and installing a new driveshaft.  I am thinking this will give me the most "bang for the buck" especially if i drop some 373 gears in at the same time.  The engine is fairly stock except for the Holley 600 CFM 4 barrel and Edelbrock performer intake.  Thoughts?  Thanks!  

Yeah I did exactly what your contemplating on my 65 fastback which was a C4 also. I wouldnt necessarily say the biggest part was the clutch pedal but it wasn't easy. Only for the reason that getting under the dash and working there is not the most comfortable. I am a pretty good size guy and you wind up doing most of it while on your head with your feet in the air. After that you have to put in a new clutch mechanism, either hydraulic or cable. Knowing what I know now, I probably would opt for the hydraulic unit since it makes it easier to put a brake power unit in later. I also put in 3.8 rear gears and I think they are ideal with the overdrive on the later T-5. (I bought a new one from the tremec dealer). I had priced out all the parts and got a kit from Darkhorse Performance(although they dont offer them anymore). Included in the kit was a McCloud clutch which I later found to be a little stiff and would recommend the dual friction unit from Centerforce to you to reduce pedal force. The new driveshaft was simple, you can reuse the C4 yoke but they recommend the longer one for the t-5(same splines). The shifter came up in the hole for the C4 shifter and I only had to trim a very small amount with a small grinder to clear the boot. After the swap it just transforms the car into something really cool. It has low low first gear that will really set you back and when you hit 5th the car just quiets down and cruises so nice that I can talk with my buddy all the way to a show(even with flowmaster 40's) with no problem. It just brings the car into the 21st century in my opinion. You wont be sorry. Good luck  

By the way you dont have to fabricate a pedal assembly, just do what I did and buy one off of ebay that is already a stick pedal assembly for that year mustang. I put in a Scott Drake ball bearing support in it and new pads and its great. They all use the same mounting screws on the dash and firewall. You have to put it together under the dash because it will not fit in in one piece(with all the pedals installed). It sounds harder then it is.  

WOW.  Thank you SO much for you extremely detailed response.  I GREATLY appreciate it!  I am DEFINITELY going to do it.  Should I call Richmond for a complete setup with Bellhouseing etc., or should I piece it together on EBAY?  I am very good with EBAY so I am good that way..Thanks again i appreciate it!!  

Well you can piece it together but then you dont get the attaching hardware,,,no big deal. Keisler also has kits I believe and probably a few other places. Mustangs plus has the individual pieces also. You might save going to ebay, just make sure you know what your getting. You will love the new setup.  

Awesome.  Thanks so much.  I do appreciate it.  i will post progress as it happens!  

Yeah, and if you have a particular problem Jforte dont hesitate to send me an IM here. I can usually get you back on the right trail if necessary. Its not that hard to do and there are a lot of sites with info on the swap as well. MM here did a good article that you might want to find and read. I cant remember how long ago it was though.  

Fantastic!  I will do the research! Thank you again!  

I haven't made the change, but can offer a great How-To Tech article on how to do it.  copy and paste this article to your browser or other search engine so you can read it.  
http://www.fordmuscle.com/archives/2000/09/t5swap/