[mr0077]

I had two 390 CFM carbs on mine for awhile, ran really well with synchronized throttles, crisp throttle response, great for street cruising, but seemed to run out of air above 5000 RPM. I changed to two 600's with progressive linkage and it still runs good, makes more power, and has crisp throttle response too. Did your friend have a progressive throttle setup? If not, try that first, it might fix things right up.
I would recommend vacuum secondaries for a dual carb setup. The mechanical secondaries might cause some driveability problems (you can over-carb with too much throttle, might bog at low RPM, etc.). In addition, I'm not sure whether it would be best to run 450's with a synchronized or progressive throttle setup. My 390's were like one 780, so synchro primary throttles worked just fine. The 450's would be like one 900, and might have a little too much primary throttle to be crisp on the street, so progressive might be the better setup.
Who know, lots of little variables to play with, and like you said, it is a light car so the 450's might work just right.
Let us know what the friend does and how it works.