What heads?

[71SSNova]

I keep looking for the correct dated heads for my 71 Nova, which according to Colvin's numbers book should be 3973487. The problem I'm having is the casting date of the engine is I 12 0 (Sept. 12, 1970) and the build date is 1202 (Dec.2). I believe the difference is the GM strike (Spet. 15, 1970 to Nov. 20, 1970) 1970 GM History. It is the numbers matching block. The car is a 12A date.

Should I go with a date close to the casting date or the build date?

And does anyone know when they started making the 487's and stopped the 186's? If the 186's were made in sept. of 70, would they be acceptable since I have such an early car.
There are a set of 487's on ebay right now with L2370 and L2870 dates but they are too late for my build date and car.


Thanks, Dan