Copy, you will be amazed at what it can do. You'll be tempted to put a huge round on there to see what it will do, and it will split the hell out of it. The problem is you will then be wrestling two 200 lbs halves wit a mind of their own. Be careful. I try to noodle anything that will be awkward like that and really prefer stuff under twenty inches in diameter. The amount of time it takes to get the wood out of a thirty plus inch round is not where it is at when it comes to efficiency when you could be pushing 18" rounds through the six way once and make six pieces on each stroke. Also, if you have any question about the knots or twist in the wood, stick to the main knife. It isn't worth the hassle of a stuck round or worse, a damaged wedge, just to try and see if it can do it.
390XP is a nice unit, how far you getting in the cut on a tank? Any post work done on that saw?