I have a little leak from that same location as well, but don't think it fumes me out in any way. Some day I want to tear that all apart again and take a good look at it; it seems that a metal bushing could be pressed in there to seal that up better. If you're going to keep your stock manifold you need to be sure that that assembly is working properly or you'll crack the bottom of your intake manifold. So do not just weld it up without having a provision for the plate to rotate.
On headers, not all headers are created equal. There is the common 6-1 header, which I think just about everyone here in RS has who has a header (including the one in your picture). There is also the Downey Tri-Y 3-2-1 header, which some claim (Marks Offroad for example) flow better than the 6-1. Finally there is the tuned header like MAF and SOR sell, which suppposedly are the best designed for flow and optimized for the engine performance. They are also (surprise surprise) the most expensive. The cool thing I personally like about the MAF design is that it has an EGR port so you can have a header and lose very little of your emissions components (The HAI and intake pre-heater being the only two exceptions I can think of right now), if you need to keep them. You can read more on the MAF website and marksoffroad.net website about headers, some pretty good info there.
I'm running the stock manifold still, and Ricardo and I have yet to race and see who wins!
On desmogging, I'm not sure what I gained by desmogging except for extra room in the engine bay. There seems to be no power difference for me. I do think, however, that I can attribute my very stinky exhaust to the desmog. Once I get all my smog components back together I'm going to throw it all back on and see if that's true. The other reason is that some "features" of the smog equipment is gone if you desmog. For instance, the throttle positioner is gone. Not a big deal, but with sloppy/worn drivetrain components, a worn engine, and a Lockrite in the back, it would be nice if the throttle wasn't so on-off like it is now without the VSV installed that controls that diaphragm. Just little things to think about.
IF YOU DO GO THE DESMOG ROUTE, KEEP EVERYTHING
You never know when the laws may change to disallow grandfathering, or change the cutoff dates, or you may move and will not be able to register your 40 at your new place without getting it back on the smog program. Just box it all up and store it in the attic. This stuff is very hard to find these days. 1977 seems particularly hard to find for some reason.
I have no doubt that headers help with exhaust flow and open things up a bit, but if you're always shifting early and keeping your rpm's low I doubt you'll ever notice. If you do go headers, definitely get a heat riser built or bought.