Do I keep it? Or trash it?

Don't trash it! I assume this isn't a matching #'s 340 for you, so it might be for somebody else. I'm personally not one of those "#'s matching" types, I would still go with a 360 for your build because you can pick them up cheap (there's a shortblock and a complete engine near me, short for $50 and the full for $125) and realistically everything that's better in the 340 (higher compression etc) can be changed during the rebuild. But either hang on to or just sell the 340 block as is, you can probably still get most of your $200 back out of it and more than likely there's someone out there that wants it...