ChrFab.com is really the best place I've found for aftermarket Northstar performance. I'm using a 4.0L N* for my project.
But they're NOT cheap. For what you'd spend on a performance rebuild, you could easily score a 500-600hp turn key crate 383. With just the parts from CHRFab for a full performance rebuild, not including labor, will run in the $5-6K range.
Plus, Northstars are REALLY complicated engines, with nearly twice the parts as an LS1.
Though, the aforementioned problems were from the factory and are easily remedied by paying more attention to those areas when you rebuild.
Only reason I'm using the Northstar is because I already have one...that, and I'm doing a V8 240Z...and I've yet to see a Northstar in a Z. Otherwise, I'd have just picked up a nice long block from elsewhere.