What you should do is forget about the egr valve and go to a wrecking yard. Get the carbs and intake manifold from a '72 or earlier 240Z. Using the earlier carbs will make the engine run 10 times better with more power. This will also eliminate the unwanted egr valve. If you have to stick w/the flat-top hitachi's, then you can remove the egr valve and plug the holes in the intake and exhaust manifolds. Good luck!!!
Ian makes good sense, and I suspect he also has a 73 or at least has worked on one or more. He's right... the best thing to do is to replace the flat tops... and manifolds. On my 73, replacing them is the what I'm doing this weekend. As for what happens if you don't replace them... and try to remove the egr valves... it is possible with one very small problem. If you look at the rear egr valve you will notice that there is a throttle linkage adjustment bracket mounted on the side. If you remove the the egr valve... you also remove this bracket.... let me know if you comeup with a place to remount it.
I have disconnected mine by plugging off the hoses. I also removed the Anti-Back Fire unit and pluged the places it's connected to. I do feel that the car does rev alittle faster but not much. I also removed the air pump, which might had something to do with it as well. I haven't plugged the exhaust pluming and plug it, but plan on doing this. I have also plugged the hoses for water. Flow is from the rear of the engine to the front. The return entry point is at the water neck. My problems started when my rear carb started leaking water into the intake manifold. Yes there is water running throu the carbs. I rebuilt the carbs in my garage and plugged these point too. Good thing rubber plugs are cheap. But no matter how much plugging you do... its all just a bandaid for doing what is right... meaning its time to replace the carbs and manifold with the earlier SU model. Its a fast drop of $200, but very well worth it. Z therapy has some very nice carbs, but I can't see spending $450 or more for the artwork. It all depends on how much you have.... and what you want... which is?