The only problem you'll encounter in using a late model 302 block in place of a 289 is with a manual transmission car, the 289 block has a threaded hole on the lower, drivers side for the clutch linkage pivot ball. Some of the classic Stang suppliers sell an adapter bracket to compensate for this.
In that case, I assume you'll be needing a correctly date coded block ? And are willing to pay occordingly ? They're still out there, but finding the correctly dated block can take time. Otherwise being content with any old 289 block is no different than using a later 302 block.
You can watch the ads here on Stang net (in the classifieds), post an ad there with the particular date code your looking for. Watch ebay. I see lots of them put up for sale all the time. Just be prepared for a long hunt to find a particular date code though.