where is the best place to hook up the brake booster, on the back of the plenum or off the manifold,the reason I'am asking is I put a big cam in and the brakes are not working to well. I have it hooked to the plenum now I'am thinking of using a vacuum canister if there is no difference in connections.Thanks

 

That would be your problem.
Doesn't matter where you hook line up to; the vac is the same anywhere in the intake between the TB and the valve