The 250 and 200 are much the same but the 250 is taller.
The blocks are pretty much the same from year to year, so it's probably more important to find one in good condition and low miles. That would more than likely be available in a "newer" block, like say from mid 70's and up.
There are also 2 distinct varietes of 200, one with a low-mount starter and the other a high-mount. The mid to late 70's had high-mounts and the older ones and the last ones from the 80's had low-mount starters, which is preferred because it has the same tranny bolt pattern as a small block Ford, and therefor more transmission options.