How much CFM for a 360?
The stock 360 4bbls came with an 800cfm Thermoquad from the factory... overkill?
You can't compare a spread bore to a square bore when it comes to selecting carb size. The Thermoquad has small primaries, smaller than a 600 square bore and the secondaries never opened all the way on a stock vehicle. So the fact that the ultimate CFM was 800 is pretty much irrelevant.
I have had a 600 (Holley VS), 670 (Holley Street Avenger), 725(BG Road Demon) and 750 (Holley VS- 3310) on my 360.
The throttle response and mid range reponse was awsome with the 600. It did give up enough to the 725 and 750 to feel it in the butt dyno above 4000 rpm. However, the 750 and 725 didn't have nearly as good throttle response as the 600. As a matter of fact the 725 was horrible below 2000 rpm. If it was a decision between those three carbs I would say the 600 all the way for a primarily street driven vehicle.
The most recent carb is the 670 Street Avenger. This carb has to be the closest to a drop on an go carb I have ever tried. The throttle response is as good as the 600 and I feel no difference by the butt dyno with the 725 or 750. Plus the 670 is giving me 2.5 mpg more thean the 600 and 5 mpg more than the 725 or 750. The 670 is unique in that it is a square bore (all the throttle baldes are the same size) but the primary vernturi's are noticabley smaller than the secondaries. This has to be the reason that it has such good throttle response.
Just like cams, it's better to error on the small side. A slightly small carb will give up much less on the top end than the bigger carb gives up on low mid range throttle reponse.
FWIW, my car with the 600 carb has run a very traction limited 13.7 @102 quarter with a 2.2 60' time. If I was to sort out my traction issues I could run low 13's with the 600 carb. I don't have any numbers on the others.
IMHO, I would not invest money in a new carb until get a chance to try some other sizes. The 600 is not going to cost you much, maybe a .1 or two in the 1/4 but you could definately be giving up a lot of throttle reponse an mid range driveablity not to mention the gas mileage.