As regards casting numbers for 273 heads there are several so you kinda need to see the numbers. But kinda not; if it has the small bolts it's the first generation.
My listing looks like this:
64/65, 2532080 These for sure are the 5/16 early heads.
1966, 2536178,2658234,2658920......These three all carry the same PN, and since the 920 head fits 318s ; I'm assuming these are all 3/8 bolt late style heads
1967, 2658920 (also listed for 318s)
68up, 2843675 (also listed for 318s)
I think that's right
But if you need an early head, and think you might be putting a 4bbl on some day, (assuming you haven't already),then I recommend getting two later heads, on account of that opens up a whole new world of bolt-on intakes.
But if you already have a 4bbl on her and are happy with it, or don't think you ever will put one on, then carry on.
