Tallchief - you may be saddled with load development to get the barrels shooting the way you want, maybe not.
Good luck - chopping the tubes just behind the chokes is a good idea - it allows almost a full bore sized ball,which should be around .615" - rather than having to use an undersized ball.
The undersized balls still do just fine for accuracy, though. Wads (cupped), of course will have to be purchased along with the fiber wads, unless there is a skeet range near by where people shoot 20 bores. O'course a bag of 250 is not that expensive.
Jeff Tanner will make you any size mould you need- for about $38.00 including shipping - payable by pay pal or to a California account.
Lee makes moulds in .600" and .610" I think and is the way I'd go for now. Either will work in a cylinder or improved cyl. 20 bore.