Do you want to get it down to tenths of secs or general use
The reason the poster said to use a 670 was because it's a way of sort of compensating for use at higher alt...Now if your talking about dialing it in to tenths of secs then that's a very different story
Running a larger cfm carb can allow you to run a little rich at sea level and still work at alt with just resetting the idle mix...
Back before all the computer programs to help one figure out the temp/humidity/alt, a general rule of thumb was one jet size for every 2 thousand feet of alt along with a bit of timing change for the paticular engine car set up ect....
I have not been racing at alt for years, but Holley use to have a webpage that listed recommended jet size by carb application by alt
BTW when you get down to tenths of secs you can still have problems at alt with EFI too....
Later,
Nick