I used Modern Driveline for my C4 to tremec conversion, could not be happier! Mine is a TKO 600 behind a 408, they hooked me up with everyting. I highly recommend going to a 5 speed it is the best upgrade I have done. I am currently using a cable clutch. Have not had any problems with it, but I have been considering their hydraulic unit that mounts under the dash.
I went from a C4 to a TKO600 also in my 69. Bought the entire kit from modern driveline. I am using their cable clutch and their shifter. Simple to install, and the cable is very heavy duty. Pedal feel is not bad at all, in my opinion doing the work for a hydraulic wouldn't be that much of a benefit.
The only thing I did was shim the trans mount to raise the rear of the transmission to get a better driveline angle. I had a bunch of space between the top of the trans and the upper tunnel support brace. I had about 4 peices of 1/4 steel plates laying around, and drilled 2 holes in them. Tucked the trans up nice and tight and corrected the driveling angle.
Make sure to pay close attention to what you want for gear ratios. I went with the standard gearing TKO600, which had a .64 overdrive. After having it installed, I really wish I went with the road race version which had a .8(something) overdrive.
If you go with a T-5, you will have even more clearance in the trans tunnel. This swap was some of the best money I ever spent on the car.
Thanks for the information. It currently has the 4 speed toploader set up, looking for the additional overdrive for criusing the highways. Will update after the install.
Thanks for the feedback. Slid