I beleive that 400 is a 1970 from a full size pontiac with automtic transmission and a 2 brl carb, Those #16 heads would be a good match and could have been from that engine. If so they would have been the small valves and 290 hp.
You can build a very nice engine with that combo.

Are you sure the 350 is a 9790070 and not 9790079? The 79 is for a 350. WV 350 would have been 1969 330 hp out of a tempest with manual 4 speed.
What are you trying to build? Street, strip, driver?
IMO, I would go with the 400 block and 16 heads. Once you decide how your going to use it, then start thinking about cams. Compcams has a long list. A stock cam will be much different than a high perf cam.

Here is a link with the info on heads, blocks etc.: http://www.teufert.net/pontiac/page2.htm