Slant 6 Help

What size exhaust does it have ? is the muffler plugged up ? what spark plugs are in it, I would think you have electronic ignition.
Make sure your heat riser is not stuck half closed in your exhaust manifold, this will make it a slug slow /6... doing a 2 1/4'' down pipe to a new muffler and exit with a 2'' will help if your 73 /6 has the idibity 1 5/8 exhaust.. does the /6 have hydraulic lifters (non adjustable)
or solid lifters with adjustable rockers ?.. a good valve adjustment will go a long way if they are in need of adjusting.... New plugs and wire along with setting the timing..
Make sure you don't have drag, wheel bearings, U-joints and rear axle bearings..
Doing these things help get these /6 engines moving and breathing better.
Plane and simple, good tune up, no drag from worn parts, and take the potato out of your exhaust (check that heat riser) it will hold you back)..