I have built a AK74 with one of the Classic AK74 barrels. It built up OK and is the example in the pic you see on their site..., an AK74 underfolder. The webmaster contacted me to get a pic as the customer response thing they started.
Too bad you missed the recent K-VAR barrel sale. They had milspec Chrome hammer forged AK74 barrels for $150. Not as cheap as the Classic but for $50 more, worth the price.
You have to be sure your journals are not too tight because the US barrels are definitely softer than milspec. Some folks have bent them at component install or final barrel to trunnion install. I have not put a lot of rounds through it (maybe 200) but it is accurate and shoots fine.