I started looking through my info in the Delco parts and service manual .
One of the things that jumped out was the approx year of the replacement manifold. It appears it came from a 1980 year or close to that group. The EGR valve I believe was not on earlier years, but if you disable it , it would not affect performance and if anything it would prevent the possible hesitation problem. If there was an older manifold without the EGR valve on it the alignment pin problem would probably not be an issue.
If you chose to , you could also remove that vacuum tree and install a plug in there since you won't need the EGR valve to work. Is there any possibility you could find an intake manifold from an older year. I might be able to provide the exact year the manifold in question came from if you could provide the number on the side of the main body of the carburetor.. That is if you really want to know or don't know. Not important if you cannot use it.

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Thanks again Hanky,
i don't have another manifold to use and getting hold of one given all the lockdown we are all in will make it difficult to get one soon I'm afraid.
I agree that it must be a more "modern" engine that has all the EGR ets installed . I'll think on the next best step to take, maybe even get a 2bbl to 4bbl adapter for now so i can at least get me a running engine which will move the project on somewhat. They do look a little restrictive but i'm not after a igh performer, just a cruiser really.