HP2 designation as "typical" for 6 pack engines is a myth as far as I am aware........HP designates an HP engine assembled on first shift....HP2 indicates second shift assembly. I can think of no valid reason to only assemble 6 pack engines on 2nd shift. I assume these engines were assembled throughout the work day regardless of 4 barrel or 6 pack configuration. However, I may be proven wrong. It would not be the first time.

For the OP.....accepted date range for exhaust manifolds is several months to just a few days prior to the vehicle SPD. I don't think you will correlate it very well to block casting dates as they do not seem to align with SPD's.