Interesting find! And very good visual renderings Franco!
Of course given our modern desired methods instead of the "Manhattan proposal" a toned down but still effective middle ground is met, I envision that a stone arch bridge at the proposed location A on the northwest arm, or even location B would be very fitting with the character of the neighbourhood and provide a much needed crossing over the arm and relieving the Armdale roundabout. I figure two lanes for general traffic and protected bike and pedestrian pathways would be helpful to encourage active transportation.
Alongside a re-imagined south end crossing, I would personally feel a trench tunnel similar to the Ted Williams in Boston Harbour would make the most sense.
Dredge a line from the industrial lot next to imperial oil terminal at the end of NS111 in Woodside make a thirty degree turn to have it lead up to the parking lot next to Tim Hortons on Barrington Street and a ramp from Marginal Road for the Port truck traffic. Lay down the tunnel in the trench in re-fabricated concrete sections, sink them down, weld them together, pump out the water and bury the tunnel under dirt on the harbour floor. This would not be accessible to pedestrians.
It would be a six lane tunnel with four lanes leading to Barrington and two for Marginal. From Barrington two of the four lanes will be BRT lanes.
The Harbour bridge commission is already looking at a "new" Mackay bridge alongside the existing one that will be six lanes with protected shared bike pedestrian pathways, once completed they would teardown the original "New" bridge built back in 1970.
A twinned Macdonald Bridge would actually be very helpful in relieving congestion and accommodating the planned BRT.
With these hypothesized twinned Macdonald bridges having a combined six lanes, two can be reserved for BRT alongside the already existing cyclist/pedestrian pathway that exists on the OG.
My proposal in summary would consist of four new bridge structures to be built. Increasing the capacity of two existing crossings and building two completely new crossings. It seems fairly modest considering the tens of billions being spent of transportation mega projects around the globe.