My understanding is up to 6 months prior to build without point reduction. Most people try to stay within 2-3 months.
I was thinking approx 4-6 weeks prior to build date, so late February thru mid March is what I would guess. Could be a little closer to April and still be ok. BTW, no point deductions for date codes in Buick GS Nats concours judging, but other big shows could be different.
My early 71 (09A) GS Q-jet has 1970, which is closer to 3 months. The iron parts were all about 2 months out with dates G1,G3 heads, F31 intake and G2 block. I don't recall the manifold dates off the top of my head. My rule of thumb was longer dates for parts brought into the plant. That fell apart when I found late August dates on my wheels... so generally anything prior to the build date out to around 3 months is going to be correct, then beyond 3 months the less likely. Yes, there are exceptions...
