600 CFM, all I need. Check my math!

Here's an article that might be of interest...

Selecting Right Carbs: How Big Is Too Big?- Car Craft Magazine

In it, they state what I have heard several times and have found to be true as far as using a large carb on a dual plane intake...

"Another important factor influencing carb selection is intake manifold design. With its divided plenum, a typical dual-plane intake provides a stronger signal to the carb than a single-plane manifold. Grant says a 350 Chevy with a dual-plane intake can usually use a 750 carb; if it’s running a single-plane, a 650 carb will probably get the car down the track quicker. One way to crutch a single-plane/large-carb combo suffering from insufficient signal is to use a four-hole carburetor spacer."

I still think you're fine with the 750 and it sounds like you are headed in the right direction with tuning! As far as a gauge, I would not over think it...just get a basic one. Advance auto has one for about $25... Actron part number CP7803. Most of their stores stock it, it's got a large, easy to read dial...and who cares if it's not a $$$ calibrated unit, you are tuning for maximum vacuum anyway.