You can't make a comparison between a spread bore thermoquad and a square bore carb when it comes to cfm ratings. The Thermoquad likely has smaller primaries than even the 570 Holley and a stock 340 would never have opened the secondaries fully to realize that 850 cfm rating. If you have one it's certainly a good choice but just don't use it's cfm rating as a guide to buying a square bore carb.
I have run a 600, 670, 725 and 750 on my 360 which is far from stock (~370 HP). The 600 would give up a little on the top end (above 4500 rpm) to the 670, 725 or 750 as measured by the butt dyno but it had it all over the 725 and 750 in low/mid rpm throttle response. The 670 is ideal, same low/mid response of the 600 and no difference as measured by the butt dyno on the top end. Also, the 600 and 670 get about 5 mpg more than the 725 or 750.
If you are buying a new carb the 670 would be a great choice. If you have the 570 it will work but you may give up a little on the top end but for a street car you will likely never notice.
