Need More Vacuum

Here's my build:
318LA
.030 inch overbore
1.88 and 1.60 intake and exhaust valves put on stock head
Purple Shaft 340 specs cam
4 barrel spreadbore Eddy intake manifold
Rebuilt Holley 650 4175 spreadbore carb
Hedman headers

Currently I'm getting 7inchlbs of vacuum off of my manifold. I need at least 14 in order for my power brake booster to work. How do I increase my manifold vacuum?

Thanks