New question prop selection

Hi again,
I have a new question for any volvo guys, The prop that is currently on my boat is a 15 X 17 and the pitch is not large enough. Heres my problem after doing a search on the net I found that my model stern leg volvo penta 280 has two output shaft sizes the first is a short shaft 4.705 inches and the second is a long shaft 5.375. Now they recommend V6 and V8 should only use the long shaft, Mine surprise surpise is a short shaft with a 350 V8 driving it. Can I somehow modify my unit to become a long shaft model, the short shafts maximum running prop size is 15 x 17 where as the long shaft can run a prop up to these sizes a 15 x 21 or a 16 x 23. Can anyone help
Thanks Simso