HEY SPARE..... HELP!!!!! and others

[charlie_the_tuna]

so the prop that was on the boat was a little banged up. i spoke to the guy and he got me a replacement prop. i drove up there today and to my surprise, he had a brand new (aluminum) prop waiting for me. well, my surprise quickly turned to despair when i went to install the new prop. so everything was perfect. i popped off the big washer at the base of the prop shaft, cleaned it up, greased it and put it back on. i slid the prop on the shaft, and...... BOOM!!!!! fit like a glove. excellent! so i cleaned and greased everything else and started to screw the castle nut onto the shaft. well, i screwed it all the way down to the taper and soon discovered the prop hub was shorter than the original. so now, not only will the prop play on the shaft a good 3/8", but the castle nut screws down past the cotter pin hole, so when installing the pin, it will be above the nut and do basically nothing.
now i'm assuming there is some type of shaft washer to either washer it up from the bottom, or washer it down from the top. what are my options here, and which is the way to go?
HELP?!?!?