Best CFM carb for a 416????

Decided to build a 10.2:1 416 with a forged assembly. Eddy heads flow 275/200 @ .600 My cam will probably be a .555 solid flat tappit cam. Going in a 3100 lb car with 3:91s and a 3500 stall. Gasket matched Performer RPM air gap. Wondering if an 850 will be fine or should I get a 950? Also curious if a carb spacer will gain power and if so which one. Once i get it together I have some free dyno pulls at a buddys shop some weekend. He is a chevy nut so I want to show him what a small mopar can do. He is the same guy that said 350HP on my 340 that pulled 428. He is guessing 450 on the 416. Im shooting for 500 or a touch better