I installed one of the repop booster kits in a friend's Challenger about a year ago, and I still haven't got the bugs sorted out yet. We started out with an absolutely terrible mushy brake pedal and have improved things up to a somewhat soft, but effective brake pedal.
1970-74 Mopar Cuda E-Body & 1971-74 B-Body OE Style Brake Booster & Master Kit | eBay
Looking at it in person, the unit from Pirate Jacks appears to have a nice build quality, it's made in Taiwan and not China, and there's actually a manufacturer's logo on the box.
After bleeding the brakes a bunch of times, then buying a pressure bleeder to bleed again, then switching to conventional fluid from silicone out of desperation, I still had a lousy pedal. I finally figured out that much of the problem was a poorly designed master cylinder. Installing a new Raybestos master made a big improvement, but it's still not right when compared to my friend's other car with factory power brakes.
Next up is to change the booster push rod adjustment as far as I can go to see if that helps any, though I have a feeling that I'm almost at the end of the travel now.
I can't condemn the booster yet, but there's a possibility that it's not designed correctly.
It takes a bunch of modifications to make the B/E booster fit in a C body. The pedal push rod is too short for the C and needs to be cut and extended. The booster to firewall mounting stud pattern is smaller on the B/E booster, so new holes would have to be drilled in the firewall and pedal brackets under the dash, and who knows if the studs will even fall in areas where they will hit the brackets.
You're better off finding a C body booster and having it rebuilt. You might want to try CTC's Auto Ranch or French Lake Auto Parts. It seems likely that one of them would have one, unless they sold them all off to be changed into "Hemi Boosters".
Jeff