On a 505 inch engine I'd use the 4500 flange intake since it has a much larger plenum. Then I'd buy a new carb with the dual flange base plate such as the Holley Ultra XP. An 850 DP would probably be the right carb for a 505 engine but it depends on how hard you spin it.
You might need a short plate between the carb and the intake to allow the throttle linkage to clear. I make a plate that does the trick but you can also use a simple spacer or grind some clearance into the intake.
I am using the new trick flow 240 heads so maybe the 4500 with the larger plenum is the way to go. The heads look like they will breathe pretty good.
