I ran a Versailles rear axle for several years, my park brake was operational but rarely used, never once did I feel that the rear brakes weren't holding, always seemed to be working fine. 

I ran a Versalles rearend for years and never had a problem, I changed to an 8.8 because the unsprung weight was much lighter.

You have to keep tension on the parking levers for the spring loaded caliper pistons to adjust for pad wear. The best way is to hook up your cables to the rear versailles cables. As ms mentioned, the force used to set the brakes is pretty tough for minimal parking brake effectiveness.  You really need Arnold swartzenegger arms

Cobra rear brakes will set you back around $600 if you buy my bracket kit with ebrake cables and a complete set of new/rebuilt cobra calipers and rotors from autozone, for example.  I have a parts list that shows everything if you want to email me and request it.  steve@mustangsteve.com
That cost includes new wheel bearings as they have to be replaced when you remove the Versailles brackets and iinstall mine.
If you can score some COBRA calipers used for cheap, then it all gets cheaper.
It was mentioned that you need some tension on the cables in static position to keep the calipers adjusted.  I did not find that to be the case, but it usually winds up that way trying to provide enough stroke to apply the ebrake..  Any pre-tension on Versailles caliper cables will cause the brakes to drag, causing "blue rotor syndrome" if they are too tight.  The levers at rest are against a stop and that is in fully released position.  And movement from that stop starts applying the brake.  That is why it is so hard to get early Mustangs set up with those brakes.  They either drag or they do not have enough stroke in the cable to fully apply.  The levers go from static position and movement of the lever only goes so far until they are stopped at the fully applied position.  There are three ballbearings that ride on a ramp, each spaced 120 degrees apart.  As you move the lever, they go from static to rolling up the ramp to fully applied.  Further tension on the cable does nothing more once they have reached full application.  That is when the "everything in 100% perfect condition" comes into play.  The pistons have to be adjusted so the pads are very close to the rotors, or that small amount of movement provided by those three ballbearings will not push the piston enough to make the pads contact the rotor.  And the ebrake self-adjusting function inside the caliper needs to not be frozen up so it tries to rotate and adjust itself on every ebrake application.  It is important to apply the ebrake alot to keep them working, if they work at all.

Same goes for COBRA or other late model Mustang rear discs ebrake.  Use it or lose it, but at least they work well when properly installed.