We have been working on this problem going on three years now. 
We finally have a viable intake manifold that will fit under any cowl.
Thanks to Tom Kendall who made the pattern and had this cast.

This is pipe and not tube. The inside radius is two inches
but I am pretty sure three inches will work. Note! This is 
not the center line radius as RB Wagner uses the inside radius
for pipe. The 1-1/2 size is actually 1.90 inches in dia. 
The wall on the stuff I have is .125 which is easy to weld.
The ID is therefore about 1.6 inches.

The other size pipe the manifold is designed for is 1-1/4.
The OD on that is 1.65 with a 1.36 ID more or less.
The wall is again .125. The wall is thick on tight radius pipe
because other wise it would stretch too much and perhaps crack 
or splitwhen bent.

RB Wagner might have some three inch radius .065 wall pipe
that would work with this manifold. You would then get a bit more
area for a bit more power. Also smother air flow.

Paul Lamar
 
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