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

ok I know some are going to disagree with me but this is the way Holley decided to set their base jetting. Holley used a slightly better than stock Chebbie 350 to set base jetting. Your running a 273 which is 77 cubes smaller. I'm willing to bet that the 650dp is a little on the fat side of things. The 650 should work great on there just needs to be jetted right. Also, the accelerator pump squirters may need to be changed to a smaller shot if your getting too much fuel on acceleration or even run a smaller pump cam.