I use early corvette exhaust manifold gaskets, easier to use because they're in 3 pieces per side. I've changed engines in the /forums/images/graemlins/burb.gif a couple of times and have always been able to re-use them. As for the collector flange check out these part numbers from summit.
(1)FLO-15922 Kit Header Collector Ball Flange 3.5" Coll. Onto 2.5" Exhaust System $58.39 B/O
(2)FLO-15923 Ball Flange Collector Kit, 3.5 in. Collector / 3.0 in. Exhaust, Pair $58.95
(3) FLO-15924 Ball Flange Collector Kit, 3.0 in. Collector / 2.25 in. Exhaust, Pair $45.95
(4)FLO-15925 Ball Flange Collector Kit, 3.0 in. Collector / 2.5 in. Exhaust, Pair $45.95
I couldn't find pics, but it's a ball and socket fit like on cats.