Inliners International Forums General Discussion Engines General intake manifold design question?

I am in the process of converting a 3.0 Mercruiser to replace the Chevy 153 in my '47 CJ2A. I'm making an exhaust manifold out of a set of SBC hugger headers, which seems pretty straight forward. I have a 2-1/2" 16 gauge U-bend which I'm planning to use as the basis for an intake manifold. But I have a few questions for you more experienced folks going in:
1. Most of the factory 4 cylinder manifolds I've looked at while searching for something I could adapt have a "box" area directly under the carb with smaller channels or tubes connecting that to the flange. Is there as reason for the extra volume directly under the carb? or could I just mount the carb to a plate on top of the U-bend? I will need to press the ends into more of a rectangle where they meet the flange to line up with the shape of the ports. I could weld up a "box" to mount the carb on, but that makes the fabrication more complicated. A big part of my challenge here is getting the intakes in-between the exhaust tubes.
2.I'm planning on running the two barrel marine Rochester, (with a couple of minor mods like a vacuum port and a manual choke), How much effect does the orientation of the carb have on performance? I mounted the Weber 32/36 perpendicular to the head on the 153, and it works well, but I did that primarily because it is progressive and I drive most of the time on the 32 mm barrel so I didn't want it biased toward the rear intake port. The Rochester is not progressive so would it make any difference if I mounted it parallel to the head? That makes the linkage much simpler, and helps a bit with fitting it in between the exhaust tubes.

One member here modified an Offy intake from the 6 cylinder and used that to fit the 153. It would be a lot less fabrication and probably a better performer than a tubular intake. Then just make a plate to adapt your carb to and you're done...!

Or you could make a carb box and use a Webber intake If you have the side space for something like that. And I have those intakes.

He mentioned he is putting this in a Jeep, so I bet he's got plenty of room...

I have started to make a plenum box for one but I have yet to get to finish it.

I'd like to see it when you are done.

I started a steel one then shifted gears to do an Alum one instead which could be bolted or welded to the intake. But I have yet to decide on the top plate 2b or 4b. Then I get to see how fast it gets copied like my other stuff. lol
But i will let you know beater

I have manifolds for my 153, which I've been driving. The issue is the 3.0 Mercruiser which has 8 individual intake and exhaust ports which are much larger, and shaped differently than the 5 ports on the 153, three of which combine two cylinders. I'm just trying to sort out some of the basic parameters. Like can the two barrel carb be parallel to the head like it is on the marine "log" combo manifold without losing performance or efficiency, and is a "carb box" really necessary as a larger chamber before the leaders into the head?

Yes

I had thought of doing some casting patterns for intakes for those heads but didn't know how big the market really was for that later head...

I have sold two of the webber 4cyl intakes. I made a spacer to use onto the 4port head. And so did one other guy I sold a intake to. The bolt pattern If I recall is the same It is just the port width that is really the difference.