The carb I used on the Kidnapper Van (318) was 780 CFM. It ran fine. I doubt the secondaries ever did very much, but since it was a vacuum secondary carb I could get away with it. Probably wouldn't have done so well on a mechanical. Also, PAINTANK... I just thought of this and figured I'd mention it - be conservative when you're tightening up the bolts on that adapter plate. I had to buy mine twice because I cracked it when I overtightened the bolts. It didn't take much either.
I've got two of these jewels and both were used as such.They're good for being in a bind, but little else.