Residual pressure valves are recommended, but the number of drum to disk conversions that omit them, without even realizing, is somewhat staggering. You would want a residual pressure valve and you'd probably want one inline to both front and rear circuits if using a non-drum master (this one appears to be disk / drum - so make sure there is no residual valve in the rear circuit already from the master cylinder). All drum / drum systems I'm aware of used residual valves @ 10PSI +/- to both circuits, but I'm not a "Mustang expert" so, take that for what its worth. You would NOT want to retain the combination valve however (looks like a standard Pv2 unit) - It would direct and delay pressures inappropriately for a drum / drum system - that being the case, you'll need to re-plumb your brake lines anyway if you move to disks later.
A note on residual pressure valves - those suckers leak. I mean, perhaps not all of them, but I've found them to be yet another weak link. You might look into an appropriate thread sealant - its the only way I've got them to hold, you shouldn't have to technically, but such as it is.