Get your float needle and seat from Yamaha, they're the best and not outrageously priced .....
https://www.partzilla.com/product/y...?ref=0c2514270f638cfd80aa2c78e639f3d5a7971cda
As mentioned, no need to replace the original float bowl gaskets unless they are bad, broken or torn. In my opinion, you can't do any better than those originals. I still have them in my '78 carbs and they work fine. Your leak is because the fuel level is getting too high in the carb, not because of the gasket. It would probably even leak with a new gasket under this circumstance. The proper fuel level setting puts it below the gasket so it's not constantly soaking into and through it.
The fuel level being too high is related to the float or the float needle and seat assembly. Either your float has gone bad (sprung a leak), it's set wrong, or the float needle and seat assembly has dirt in it (won't close and/or seal) or is bad. Instead of spending money on gaskets you probably don't need, you would be better off putting it towards an external float level checking tool like this one. You will use it many times in the years to come .....
https://www.ebay.com/itm/Carb-Float-Level-Tool-Yamaha-XS400-XS500-XS650-XS750-XS1100-XZ550-77-83/273099883552?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649