Carl is right on. For 1970 they switched to tapered-seat plugs. Looking at your engine there are a lot of things that show it to be a 1967 or prior: the open PCV system with the breather type oil filler cap, the dipstick location shows the block to be a 1962-67 Chevy II / Acadian, the style of fuel pump is 1966 or earlier style, and the center sump is again a 1st gen Chevy II / Acadian. Still that doesn't guarantee that the head has not been switched. The earliest 194 heads (1962-64) were the only factory heads to use siamesed exhaust ports. They also have small combustion chambers to maintain compression with the small displacement 194. The oil filler cap is forward on the valve cover, which is typical. The earliest versions of the  194/230 family had the oil filler cap more towards the center of the valve cover.

I have a 1968 250 block using a modified 1984 292 head with the hardened valve seat inserts. The original 1968 250 head is sitting in my shed, along with the stock intake & exhaust manifold. So, tapered seat plugs started for 1970 models, hardened valve seat inserts started for 1973 models (although unleaded fuel as a requirement didn't start until the 1975 models with catalytic converters).

 According to the casting # on your block it appears the block is a 1973-79 Chevy 250. Casting #s can be found online at this link: http://www.inliners.org/  I didn't find anything on head casting #s there unfortunately. You might also try contacting Tom Langdon of Langdon's Stovebolt Engine Co. http://stoveboltengineco.com/