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That's right, the full shrouds will do an adequate job of keeping the mast on the boat's centerline, as long as the pivot pin is installed at the step and both shrouds are at or near sailing adjustment. Because of the convex deck curvature between the mast step and the chainplates, the geometry is such that the full shrouds are a little loose when the mast is horizontal, but automatically tighten up as the mast approaches vertical.
A simple gin pole usually attaches to the mast at or near the step. Some people even use the boom as a gin pole, but that requires attaching the gooseneck to the other side of the mast from its normal position.
Maybe this is obvious, but you don't want to ATTACH the forestay to your A-frame or gin pole (see illustration). You do want to secure it so that the point of contact can't shift up or down, but eventually the forestay wants separate and go forward unimpeded.
What is probably not obvious to most folks is the fact that a simple gin pole like the one I've illustrated is actually stable as long as the mast points at a spot aft of the attach point of the hauling line. It looks like it would just flop over to one side or the other (which it will if the hauling line is slack), but again, the geometry is such that the hauling line needs to be longer if the gin pole is off to one side or the other, so it will always seek the shortest distance to the masthead when made taut, and that's where the gin pole is vertical. In other words, while an "A" frame is obviously very stable side-to-side, you don't really need that stability.
Fair winds (one of these days),
Greg
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Clipper Marine Forum
A Frame -- Vince -- 2/15/08 11:18a.m.
Raising a stable mast -- Greg CM26 Seattle CLM005771173 -- 2/16/08 12:58a.m.
More mast stepping details 
-- Mike Laine CM26 San Felipe, Baja California, Mexico -- 8/27/08 4:22p.m.