on my '67 RS. I am swapping my 2bbl intake manifold out with a 4 bbl manifold. I plan to keep the original with the car. My question is this I found a 4 bbl manifold with an accurate casting number for a 67 for $150. There is also one for $90 same casting number but different date. The one for $150 matched the build month of my car. Would not be correct since both are dated Oct 67 and the manifold would have been casted earlier.

Any reason to pay extra?

Are you sure regarding your build date? Reason I ask is I thought that 1967 Camaros were only made till Aug/67, then in Sept/67 the 68 Camaros were released.
You sure your car is not Oct/66? If so the Intake would be a year to late.
If you are planning to get all the correct carb/air cleaner and hot air tube etc to make it look stock then maybe the correct casting number would be desirable.
If you are going to put on a aftermarket carb/aircleaner etc, then I would put on a Edelbrock Performer or whatever is the current hot tip. You could also disguise the edelbrock Intake, by grinding of all the lettering and painting it orange. The Edelbrock Intake is worth a fair amount of HP over the GM cast Intake by the way.
Hope this Helps.

Good point regarding the date. My cowl tag says 10A. Thus it was built in 66?

Yes, If it was 10A it would be built in 66.

By the way, If you do plan on swapping make sure you keep all the bolts/clips/lines/tubes and all parts together with the manifold/carb/air cleaner etc., that way you can allways put it back if you so desire. Lots of people misplace parts, I have found putting EVERYTHING into a Plastic Tote makes it easier.

Thanks for the help. You also answered another question that I did not ask. About performance between chevy and Edelbrock. Since the casting numbers would not be truely origional then it would not matter too much that it is a "GM" intake. Thus the edelbrock is probally a better idea.

Just me, but I'd use the GM intake, if the rest of the car is kept looking original.