Why not buy a pair of repro inner rocker panels (I've seen pairs as low as $160-ish?), trim yours back to good metal wherever you need to, and then just cut chunks out of the repro set to patch in wherever you need to patch? It would be the correct gauge, the correct shape, and would save a ton of time compared to shaping your own patch panels from flat stock.

Sorry I do not know what the correct gauge is. If nobody here knows for sure, you could contact one of the repro companies and ask them the gauge of their repro inner rockers.