I cant help with anything fitting since I know jack about 620s, but a +0 rim and 4.5in of backspacing would make the rim 9J and not 8J. +0 means that the hub face is directly in the center of the rim width. If there is 4.5 behind the hub, then there is 4.5 in front of the hub. 4.5+4.5 = 9.
Good luck on the hunt for the info. Im sure one of the many 620 guys will be in here soon.