A +1 on the tape around the base to center the case while fireforming.
On expanding the mouth. Just fire form it instead of using expanders. Put 5-8gns of a fast powder in the bottom of the case, cover with a piece of tissue paper packed tight, then fill with cream of wheat or grits and add another tissue plug on top tamped in tight. Then chamber and fire. It will expand the neck without any problems. I've done this with 30-30 brass to make 375Win brass and ended up with a longer case than trying to use a series of expanders. I also lost several cases trying to expand that far, but none fireforming.
Just make sure that your barrel is at least 18" long if you are calling it a shotgun. I would have the barrel marked 410 2 1/2", or similar designating it a 410 shotgun.
Should be similar to the 12ga from hell project. It uses brass cases loaded to a much higher pressure, but is still legally considered a 12ga shotgun by the ATF because it will still chamber and fire any 12ga round.