tune up

Hi guys , need help with my tune . My project is 98% done (thank goodness) but now gremlins are popping up . '72 Scamp, 383, 727. 9" with 4:10's . anyway, since I installed the 3" exhaust I am having a skip at idle . It seems to smooth out above 2000 rpm and the pipes are black and sooty . Holley #3310 vacuum secondaries with #72 jets front and rear . Vacuum advance distributor with 24* advance and 5* at the crank . Vacuum leak , float setting , power valve , acclerator pump . Ran great with open headers and/or flowmasters . Any help will be appreciated . Thank you in advance , Tom