There's a clip from today for the carb size debate. That's a 250 with the Monojet (and small factory air cleaner with a filter I'm sure I probably need to change). Watch the vacuum gauge in the middle of the screen; it has to be wound up pretty damn tight to really make any vacuum at WOT (the tach is obviously not working).
Headers will probably help some, but the reality is that you are going to need to be winding it up pretty tight to really see any meaningful gain (IMO).
Another thing is that the siamesed intake manifold and head design is really pretty awful, as I stated earlier. Think a big single-plane intake on a stock 283 would be a good idea? Probably not. A six doesn't have individual runners at all--not even ports! It really works way better than it should, if you think about it. Because of this, I would think that staying conservative with the intake and carb to promote good driveabilty, low-end power, and economy would be preferable to going all-out for a slight HP gain where you don't really even drive 99% of the time (or at all, for some people).
As for the HEI changing the color of the exhaust, the ignition only ignites the mixture that the carb feeds the engine. Too much fuel is also bad for everything.
Just my 2 cents.