The vacuum for the distributor on my 1978 300 I6 is hooked up to the manifold vacuum tree, shouldn't this be hooked up to ported vacuum off the carb. or something?

 

Mine was, then I moved it to the vacuum tree and it improved my throttle response. This was a non egr carb set-up. timing at 10 btdc. however the timing might be off, as I mix-matched every year known to stick it into my 79 bronco. I would try sticking a pipe cleaner up the DIST vac. port and see if it clears up.