how large of a carb can perform well on stock 273 commando ?

I used to run a 650 dbl pumper on one of my 273s. 10.5 motor with a slightly hotter crane cam and headers. It worked ok after I got it tuned in. Definitely needed to be leaned down though .....
Later I put a 600 on it thinking it might work better but it was not a noticable difference.