If you have been following my exploits over the last year or so you know I bought a brand new exhaust system from Waldron Exhaust. It took a few back and forths to fix some issues with the bends in the pipes (Waldron's was absolutely fantastic to deal with and I highly recommend them!) yet even when I got it all sorted from a fit perspective I had a hell of a time getting all the joints sealed.
I ended up welding the pipes into the muffler which obviously took care of the leaks in those joints. Moving forward, I used liberal amounts of Walker Muffler Seal (black, remains flexible to some degree) on the joints under the floor at the front of the car and was left with a small yet annoying leak primarily on the passenger side where the manifold mates to the head as well as at the (horrible) donut gasket. I found a fantastic crushable graphite manifold gasket recommended by my local NAPA dealer which I installed along with a freshly milled manifold (torqued to the standard 25 lb-ft), and then I gooped the donut on both sides. First, I gooped the donut and manifold, assembled them and allowed it to sit overnight. I then installed the manifold, gooped up the flare in the pipe and the mating surface of the donut and assembled them.
After leaving them overnight I just took a ride and all I heard was the sweet sound of properly sealed exhaust! Woo Hoo!

Now I am ready to order the MSD Atomic EFI next week. It requires a completely sealed exhaust to prevent air from infiltrating the exhaust upstream from the O2 sensor, and now I have full confidence I can move forward. I will post my experience with the MSD Atomic EFI once I get it installed but I expect phenomonal throttle response and somewhat improved economy along with a boatload of newfound power (the secondaries on my carb are horribly tuned and I never bothered to deal with it as I have intended to install EFI).
Resources:
Manifold Gaskets:

http://catalog.remflex.com/OLDSMOBILE_Header_Exhaust_Manifold_Gasket_p/11-003.htm
Waldron's:
http://www.waldronexhaust.com/