I have a '75 F-350 with a 4.9 (300), and was hoping to swap the C6 with a ZF S5 manual.

Has anyone done this swap?

 

I have done it on a 1989 f250, not a 75. I can't say specifically what you will run into, but generally speaking, the zf was about 1 inch longer than the c6 I took out. My truck already had holes punched in the frame to compensate for this. I used the zf driveshaft, but some people re-use the c6 driveshaft and say it has enough travel to to handle the 1 inch difference.

Your biggest problem is going to be the clutch. The zf bellhousing is made into the tranny, and they are all hydraulic clutch. So you are going to have to mount and adapt a clutch master cylinder on the firewall, and get that hooked up to the clutch pedal under the dash.