First of all it's not uncommon for WPs to start to leak after a leaking radiator or radiator cap has been replaced. This even happens on cars that have been driven right up to the replacement of these items. It shouldn't make any difference whether it's a closed or open system. When you bring the system up to proper operating pressure it will fail at the weakest link, if there is one.
Re the bolts, I'm not sure on this, depending on which engine you have, I believe, I'm not positive on this, that when you remove that WP bolt on the passenger side of the car, that the arm that runs off the cam and operates the mechanical fuel pump, can drop down, out of position. This may interfer with replacing the bolt. Again, not positive on this. Hopefully somebody else with more knowledge on this can chime in.