[Firebirdjones]

First, on the EGR intake dilema, there is a thin plate that is made to go under the carb and block that passage off.
It used to come in kit form with 2 gaskets (top and bottom) and the thin plate is made of stainless, but most autoparts stores wouldn't know what you were asking for, not only that but I doubt any of them would even carry it anymore. It used to be a simple rack item 20 years ago.
I have that setup laying around here, if I can dig it out I'll snap a picture of it. The other option is to drill and tap the intake with a pipe thread tap, and counter sink a pipe plug into the EGR hole. I've done this on a few intakes for other people and it works great. Then you can use any gasket you please.


Second, your HEI vacuum advance from GM (starting in 1975) would have originally been plugged into a vacuum source that was thermostatically opened, generally off of the thermostat housing. That way it would only supply vacuum when the engine was up to operating temp. This was done for emissions purposes. Earlier years (before HEI) used a transmission control spark switch for vacuum. It only supplied vacuum when the transmission was in 3rd gear (auto) or 4th gear (manual). Neither of these setups are ideal however.
You'll find better gas mileage and throttle response by running that vacuum line to a full time manifold vacuum source.

You can try ported manifold vacuum through one of the upper ports on the carburator but that poses a few issues.
First of all, ported vacuum only works when the throttle blades are cracked open, so you won't see any vacuum at idle. Because of this, the engine will tend to run warm while idling in traffic, not enough advance at idle. The other issue with ported vacuum advance, is too much advance when at full throttle.
To explain, at full throttle you should only have your mechanical advance working for you, which is your initial timing and your amount of advance added together. The vacuum advance should be taken out of the equation. The problem with ported vacuum, is at full throttle it's still supplying a vacuum pull on your cannister, so the total timing also has vacuum thrown in the mix. People do this all the time and wonder why they have detonation problems under full throttle, and they want to blame it on other things.
However, with manifold vacuum, once at full throttle vacuum is non existent, which takes the vacuum advance out of the equation.

For example, if my engine makes best full throttle power at 38 degrees total timing, and I set my initial at 12 degrees, then I need to have my mechanical advance dialed in to allow another 26 degrees of timing at what ever rpm best power was found ( say 2800).
Once that is set, then play with the vacuum advance. If you hook up to a ported vacuum source (on top of carb) that vacuum advance never goes away at full throttle. So if you have an additional 15 degrees of vacuum advance, it throws your total up to as much as 53 degrees!!!
That's a ticking time bomb.
Hook the vacuum advance to manifold vacuum, so you have your vacuum advance at idle and part throttle conditions (which is the purpose of the vacuum advance anyway) but when you go full throttle it's completely out of the equation, so you have 38 degrees total as desired at full throttle, but back at part throttle the vacuum advance is working again for better part throttle drivability.

Setup this way gives you the best of both worlds, you get throttle response, cooler operating temps, and better gas mileage while putting around town, and you still have the total advance you need at full throttle.

I also suggest an adjustable vacuum advance unit. Most factory units dial in too much vacuum advance. For a heads up, the amount of vacuum advance is stamped into the flat portion of the vacuum unit just behind the cannister.
Most non adjustable units will dial in 20 degrees or more (check yours to see). I prefer an adjustable cannister, so I can dial in an additional 12-15 degrees for part throttle cruising. That's all I've found I needed once the rest of the distributor is setup properly.
Hope that helps.