If I go with the wooden brake idea, I'm thinking along the lines of a
4x6 treated beam for the leaf if I can find one straight enough.

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Dear Del,

Consider a couple of different lines :-)

The point of contact with the metal is seldom more than half an inch wide and
usually less. The only thickness consideration in the leaf is to accommodate
the fasteners.

The leaf is a LEVER. You should be more concered with its depth; 2x10 or
2x12... something along those lines.

You only need one square edge to accomplish the bend. Doesn't really matter if
its warped, bowed or BOTH... so long as you square it up before attaching the
hinge.

Drawing from the knot-hole collection at the local Home Depot, the best leaf
material is often 1x12 pine shelving... albeit cupped, bowed, twisted & warped.
Simply pick a pair of complimentary boards, REVERSE their defects, slather on
a good coat of urethane glue on both surfaces, clamp them together and secure
with one deck screw for about every 9 square inches of surface. Then leave the
thing to cure, well supported and out of the weather. True up one edge and
there's your leaf.

There's a lot less to accurately bending tin than most folks realize. The
problem is that most homebuilders have no experience doing repairs in the
field, where your only option is to use whatever is available to accomplish the
task. Bending one airplane's-worth of tin isn't anything to get excited about
and even a rather casually built Po' Boy brake is capable of producing better
accuracy than the average homebuilder can utilize.

-R.S.Hoover