I'm not sure about the bolt pattern to a 203. But the easiest way I've put a 4l60e to a 205 was a 6 to 4 bolt adapter and a spacer. Both from advanced adapters. No tearing the trans down to change shafts etc. here's a link.
http://67-72chevytrucks.com/vboard/s...d.php?t=754191
Another showing a little more detail of how it goes together.
http://67-72chevytrucks.com/vboard/s...d.php?t=754661