I am in the middle of reassembling my car and also would like to head this one off while the car is dissassembled. I have searched quite a bit and not found much, but I would like to share with others what I have found.
The biggest problem I see is mounting the clutch master, brake booster is in the way big time. I personnally would like to use an external slave, but if the headers end up hitting, internal slaves are avalable for the top-loader.
I have seen the hydraulic clutch done 2 ways in a 71-73. The first is by buidling a master cylinder mount that is right behind the P/S gearbox. Kinda hard to describe, pictures worth a thousand words so search for "hydraulic clutch 73 convertible" on the "Ford Muscle" website. That is a 5-speed conversion.
The other way I have seen it done is by making a bracket that attaches to the clutch pedal. From the drivers seat it is bent to the left. The clutch master is mounted to the firewall just to the left of the brakebooster. It is a tight fit, but it should work. The pushrod attaches to the "L" bracket. Should be able to find this one on the internet as well, I run across it occasionally.
American Powertrain does list a hydraulic conversion kit for 71-73 Mustangs. I did talk to them last year, I think, and a bracket has to be fabricated that bolts to the clutch pedal. When I talked to them about it they were looking at having that piece supplied to make it a complete kit, haven't heard and haven't had time to check. It is kinda pricey. I requested the instruction sheet to see how it installs, they wouldn't send it to me. I would like to see how they mount the master cylinder.
That's my 2 cents.
Louis