A small/thin open end wrench that has the 1 end turned 45 or 90 deg will help you. Do you have bolts or studs? If bolts which are notorious leakers, on the 1 that is leaking you might get an OE type stud that has the shoulder in the middle and this might solve it without resorting to a helicoil (do put some sealant on the inner coarse threads). You might be able to just take out the 1 bolt that is leaking & replace it w a trimmed OE stud & leave everything else in place depending on how much clearance you have. You can trim the outer fine threaded section of it for a better fit (mockup/measure/trim it ahead of time). You only need enough threads there for the nut to fit flush and any more is a waste and for sure will hit a pri tube. All BB head header bolt holes are wet so drain a gallon of coolant 1st. Good luck w it. EDIT shoot a quick shot of starting fluid into the hole to clean the head threads so your choice of gasket cement on the stud threads will seal better