There's a lot of variables that go into that question, street only? Street/Strip? Strip only/Full Race? There are several CFM calculators on the web if you google carb cfm calculator, based on cubic inches, compression ratio, cam profile, expected rpm and probably transmission type, that being said Chevrolet sent the Z/28 302's and LTI 350's both out the door with a Holley 780 vacuum secondary dual line on them and I've seen pictures of full race under 305 ci 60's Trans Am road race engines with dual Dominators on them, it's all about tailoring the carb(s) to your combination and depending on how you're going to use it, I could see an 850 on a 5.3 race engine but it'd be hard to tune to drive on the street especially with an automatic, that being said I have a Holley 600 on the 350 in my 81 with a 4 spd and 3,50 to 1 gears it runs very well all the way to 7500 rpm, sorry I don't have a definitive answer for you but there's some things to think about anyway, Later, KillerDave