For carb kits, search ebay, or I like Siriusconinc.com. Might even try Yamaha.
As for what it is, the earlier 360/400 twins (76 ~ 82) were XS models.
In 1982, Yamaha came out with a whole new 400cc twin and put it in the XJ400 Seca and Maxim models. The earlier models were SOHC and the later ones were DOHC. The DOHC models also had the alternator up behind the cylinders below the carbs instead of on the end of the crankshaft.
In 82, I think you could get leftover SOHCs dubbed 'Special' or 'Heritage Special' or the newer Secas and Maxims.
By 84, they had ALL disappeared in the aftermath of the Honda/Yamaha sales war and the US recession of that time.
Post some pictures of yer new toy, if you get the chance.
Kirk