Assuming I can find someone who welds aluminum, what are
the pros and cons of using 1.5 aluminum B18 intake manifolds
with 3 carbs and a custom made linkage?

As far as I can see:

It would look cool
it would allow me to use fuel injected exhaust manifolds
it may breath better

it would be more annoying to tune
it may not work at all

I've heard of 3 webers on these cars (and I think that
kgtrimning has a manifold) but I like SUs and dislike
webers, and I've already got spare SU sets sitting around.

An alternative would be to weld up something to an
existing FI manifold to adapt it for use with carbs.

This has been sort of discussed before but I don't remember
any distinct conclusions.
chris