I just bought a 1970 YD 400 engine with the small valve #16 heads, would these heads be good candidates for larger valves and port work or are there much better castings to work with?

The ports are identical among all the d port heads. If you had the heads fitted with 2.11 x 1.77 valves in the rebuild, they would have the same flow and performance value as a stock big valve head. This has been tested and documented, look for posts by Rocky Rotella on the performance years forum...