Is this block usable?

It's fixable, that's for sure.

I wouldn't run it as is though. It might not be a problem, but if it develops even a minor leak there it will be all kinds of annoying. And if it develops a major leak after you've already done all the work to rebuild the block and install the engine it will be a real pain to drag it all out and have it fixed.

Considering what useable 440 blocks can go for you'll probably still be money way ahead after you have it fixed compared to buying another one.