I think that you might have misunderstood something. Look up H12/4, that is what I would call a non straight brake arm, They were used on the rear of Series 'D's. They are longer than the original front brake arms and that is what you are looking for. You only need to use the curvaceous ones if you have air scoops fitted. The length is the important thing as that means that for a given force on the handlebar lever more force is exerted on expanding the brake shoes, at the expense of more movement. However, more than that change is required if you want to get rid of sponginess. Buy, or make, very robust brake cables. Two of them. Get rid of the balance beam and replace it with a separate support on each fork leg. That means that you have to have a handlebar lever with its own balancing system. Enfield or modern Japanese ones are available. Then have a look at your brake plates. Are they the original steel plates with no stiffeners? if so they need to be either stiffened or replace with thicker and stiffer ali ones. Finally, are the brake shoes touching equally all the way round? If not then if you use ali plates ensure that the central hole is oversized and then when fitting the brakes, and before tightening up the tommy bar spindle, apply the brake as hard as you can to centralise the shoes and then, while holding the brake on tighten up the tommy bar spindle.
See also Lee's comment above.
