Pros & cons to both;
* Closer to the motor is better for performance, but that will also heat up the Carb bowls more so a heat shield may be prudent.
* Farther back will allow space for Petcock (as well as room to installing Vacuum fittings), But you will need to support the rear of the Carbs to keep them from bouncing up & down, causing float bob & air leeks around the rubber manifolds.
I'm thinking a happy medium is somewhere between. More forward, for a rear mount petcock. but still back enough to leave room up front for the Vacuum fittings & possibly a heat shield, then still support the back of the carbs for good measure. Hopefully that would allow you to get the petcock a little more forward to avoid running dry down hill or during heavy braking. Also, you wouldn't want the petcock tube hitting the top of the tank from inside.
If you don't mind getting fancy with tank, you could also install an inner baffle about mid section of the lower left side, to slow down the forward fuel slosh.