Tunnel ram, vacuum tuning?

I'm getting ready to start tuning the twin 4160s but the 4160 only have vacuum out of the front block and my manifold has no fitting for vacuum.

So.... where do I drill the manifold or should I just tune off the front blocks?

I do have a wide band but I wanna tune for max vacuum before I start tuning for AFR.

or am i totally off base on how to tune twin carbs?

Thanks again.