I have a '70 Chevelle SS396 that I believe to be all original. The build date on the cowl tag is 10D. The block is stamped T1009CTW. My question is the intake manifold date G 25 9. The casting number is 3955287. The heads are J 7 9. Does the manifold date raise any red flags or is it reasonable? Thanks.

Not at all. That is not that uncommon for a date spread to be like that. Intakes were mass produced and could have easily been installed with that date.