Hobby stores sell brass tubing in sizes that fits inside the one size smaller - up to 1 in diameter, I think. There will be a size that is very close if not perfect for the cases. You need to turn a rim with base, about 1/2" high, then low temp silver solder the body onto the bases.
: J.J. Donnely's big green book on Ctg. conversions tells and shows how to do this. They will be low pressure rounds, of course, which is what the 8 bore is anyway.
; I made some extensions once, to make .45 3-1/4" out of 2.1" .45/70's with perfect results by following his directions.