Unisteer offers 2 completely different racks, the power version is based on the GM J car rack as is used in the majority of kits. I don't know anything about the manual rack except that it is end steer like a typical late model Mustang rack, although much shorter. The J car rack does result in a slight increase in the turn radius, but the stock design is bad to begin with compared to a modern car. When the procession of Mustangs on the MMA tour had to make a U turn in the street, virtually every classic Mustang had to back up for a second attempt at the turn.
I'm working on the next generation suspension and steering design (my own) based around Wilwoods new forged 2" drop ProSpindle and aluminum hub with 2 piece rotors. I designed a bolt on rear steer (drop) arm for it that has a much shorter length specifically designed to work with the shorter throw of r&p steering. The steering arms are finished and the caliper brackets are being machined this week. I made 2 different designs; one using a 12.72 x 1.25" rotor with the C5 Corvette PBR caliper, and another using the Mustang Bullitt/Cobra PBR caliper with a 12.19 x 1.1" rotor (which by the way fits perfectly with brand new pads). They are designed to use '94-'04 17" wheels without spacers, and I've already test fit a 17x9" Mach 1 knock off. The smaller rotor/caliper will fit inside 16" wheels, but I don't know if the backspacing of vintage type wheels will allow them to fit inside the fender lip.