new owner with a wheel/tire question

[okyankee]

first off, i have used the search button to no avail. haha.

i am picking up a 67 swb chevy tomorrow. it is lowered already. not sure how far. my question is, what is the widest rear tire that can go on the back? i know the bed floor can be built around it, just not sure before it hits the frame. i was thinking 20's on the front and 22's on the back. or 20's or 22's all the way around to match. thanks for the help with my first of a thousand questions. haha. i am really excited to get my truck. been wanting one for 20yrs almost.