Funny, I have the same year model 520 and one of the few places I have rust is a small hole under the battery holder. They must have had a battery that vented and driped onto the fender well.
I wonder what makes it worth extra 13 cents:)
http://cgi.ebay.com/ebaymotors/1962-DATSUN-AND-1963-DATSUN-PICKUPS-L320_W0QQitemZ180188333661QQcmdZViewItem?hash=item180188333661
If it has been sitting that long at the least you should replace hoses and Thermostat and flush the system if you havn't already. If the thermostat is stuck that could be causing your overflow.
The truck your were referring to (Pope Face) was in Canada. I believe they used the J13 engine in the 521 as did Australia, South Africa and others. We greedy people in U.S. prefer displacement. Mark was yours from Canada originally?
I have a diff from a 620 in my 520, 3.89 gears. Much more driveable then standard. Just takes more work to get it going. I use mine for short commute to work. It's fine for that. 1hr. you might want to get some door weatherstripping. The rattle must be driving you crazy.