'59 head and valve cover conversion date?

[Garry B.]

Is there a concensus on when the '59 heads and valve covers went from staggered to non-staggered? In Noland's book he gives an appoximate date of March 9,1959 and the NCRS judging manual gives a range of May 5 to May 19th as the conversionn time-line. I have a original '59 270 hp block with casting date of D99 (April 9th) that falls in the middle of these two estimations and don't know which 550 heads I should be looking for. Thanks.
Garry Barnes
18531

[Bill M.]

Re: '59 head and valve cover conversion date?

I'd trust NCRS dates, and look for 550 (all 550s have staggered holes) heads from Saginaw (year date will be just 9, not 59). NCRS data is much more recent than Noland's. Remember that head dates aren't judged by NCRS.