I have a 400 B Block stroked and bored to 505 ci.
The car will not be a daily driver. It will go to car shows and the drag strip with the car club.
I hopped on Summit's website to order a M1 intake manifold.
I have a choice between one with a dominator flange and one with a square / spread bore flange. I do not have a carburetor yet.
Both are a single plane.
Any suggestions on which one to get?
Thanks in advance for your help!

My first choice is a 4150 intake. If you have the room a 2 inch 4500 (dominator) to 4150 adapter allows you to use the dominator on a 4150 intake if you choose to go that way or you can use the 4150 carb. The adapters work very well and many swear work better than a 4500 intake.

What I do is mill 1/4" off the top of the adapters and glue (using Seal-All) a 1/4 inch of phenolic. I glass bead the phenolic or sand it so the glue sticks. I then blend the phenolic into the adapater. The phenolic is a great heat insulator and that is why I use it.

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.