Adding a power booster to a manual brake system for 73 Valiant auto s six

Don't tee off the valve cover hose. That is not a vacuum, just a "breather" that should be close to 1 atm. On a slant, the intake manifold should have a vaccum tap on the rear runner. If you have anything vacuum-operated (a 1973 surely does for emissions at least), there will already be hoses connected. Booster vac hoses are fairly large (~1/2"D). If your tap doesn't have a large nipple, you can steal one off a junkyard car, and could be any Mopar thru the 1990's or so, or search ebay.

If you don't have a booster, you can buy a new one w/ brackets and MC for ~170 on ebay. In my avatar, you can see a booster/MC from a ~98 Breeze on 74 brackets in my 65 Dart. That was before the cheap ebay ones. I used the Breeze booster hose too. If staying w/ drums, you probably don't need a booster. My 1964 Valiant is such (search post). I installed a 98 Breeze ABS MC on a 2-4 bolt adapter plate ($30 ebay). That is a 7/8"D bore, which gives an easy pedal. I went booster on the Dart because I hope to later change to front disks.