It is my understanding that if you keep your duration as close to 225 as you can the more vacuum you will make. However, you can run a remote vacuum canister with a little more duration. As I said I am running 232 duration and a 592 lift on my cam and I make enought vacuum (with a canister) to run power brakes, I just don't have any room for the setup.
I really like the rumpty rump my engine makes as well as the instant power when I want it. Lunati makes a real nice hydraulic cam setup that is just a step above the "Recommended cam" for Edelbrock heads and Performer RPM manifold. Cost is around $200 with lifters and your Edelbrock springs will probably be alright. If you go up to a higher lift cam (closer to .600) you will definitely need to change out the springs on your Edelbrock heads.
Just a side note here: I run a MSD shift light in conjunction with my MSD 6AL box. I run a 6400 RPM Chip in my box and a my shift light flashes at 6100 RPMS (sometimes when you are in a race you could forget to shift if things get a little hairy without a little shift light to remind you).
