8 3/4 help please

i am considering buying a 8 3/4 rear end for my 68 dart. the rear end originally came out of a 68 dart, and it has the small 4 inch bolt patern. i was wondering what is needed to be done to change to a 4.5 inch pattern, with the fact of the drums still being a small bolt pattern too. and is it hard to find wider rims with that pattern and save the hassle of changing over the size?this is my first car and any help would be great.