A couple of layman thoughts:Now we can see if this Ferrari engine actually fits in this tiny little Alfa 105's engine bay.]
1. Could the intake runners be radiused 90º so that the red plenums were rotated down and sit down above the heads? You'd keep the F-car plenums, the intake runner length would remain the same, the throttles could be at the front as you said, and it may clear a stock hood (bonnet?).... hmm. You'd have to take them off to get to the plugs, yes.
2. Even though I understand those blue steel tubes boxed-in are quite strong, I feel like the thin-ness of the center of that new "beam" essentially puts a structural "hinge" in the shell of this car. That's how we would describe a strong structure in one portion of a building or bridge, and a strong portion in another, but the joint between them is not near as stiff and the whole thing will tend to "hinge" right there.
You don't have the space to increase the depth of that member UP obviously. What's the situation beneath this cross member? Is that the lowest portion of a 105? The pan sits lower by a bit, then some of us hang the steel strap pan guard even lower. Could you reinforce this crossmember beam with more depth down, below the current bottom edge? You could fab an aluminum skid plate that would divert an impact away from your new pan and the cross member. The BMW E30 guys have some nice ones that are bolt-on.
Even a radiused or bowed steel shape that ties solidly into the uprights at each end. It's not like a bridge where it need strength in the middle - it's a link that wants to bend or flex at each end joint (or the center if not stiff enough).
Just ideas for discussion!