I have a '71 Dart Swinger I just put the SSBC front disc brake kit on. It came with an adjustable proportioning valve (see pic), but I'm unsure as to where to install this valve. The directions are not very clear. Can anybody help direct me? Do I put it between the master cylinder and the rear brake lines?

It goes between the rear brake line and the distribution block. Try to put it close to the distribution block.

Yes, so you can adjust it.

Yes between the rear splitter/prop valve and the rear axle in series in that line. If you put it in just after the M/C it would reduce the psi to the fronts and the rears simultaneously & you want to be able to reduce the psi to the rears only as needed to keep them from locking up & you want the fronts to lock up just before the rears on a hard stop

why would it effect the fronts?

If you put the adj prop valve just downstream of the M/C it would reduce the psi to everything downstream from there which would include the fronts

but he would have 2 lines unless its a 66 or older setup

You ARE correct, I totally spa ced that . the only issue I could see with it there in the M/C rear brake line up front would be if the reduction in the one line would possibly let the electric brake warning light in the prop valve to shift over to the low psi side & go on like it does with a leak but I ain't sure if there'd be enough of a reduction in psi for that to happen & it's beyond my skill level to go any further with that. All I can add is traditionally they are put in the rear line so I'm thinking that there must be a reason for that

I my barracuda race car I put it in the line to the rear brakes, I ran new lines and the first line from the master cylinder cam out at the underside of the drivers seat next to the rocker panel. I drilled a hole in the floor and installed it there. Actually it worked out well as it was easy to adjust there, out of the way and served as a coupling to continue to the line to the back.