I would use a .135 card rather than a felt wad. Card wads were used for gas seal, felt wads for filler. I would certainly trust a reduced powder charge from a 1/2 ounce load, whether 2400 or 296. Examining a modern plastic .410 wad will convince you that you don't need a filler. I personally would use a plastic wad with the petals shortened with scissors to reduce the leading that you will get if you use the card wads, especially if your bore is not perfect.