I have a 1991 740 wagon with around 300K on it, stills runs perfectly. I've noticed recently that a small amount of coolant seeps from the base of the metal junction that the upper radiator hose attaches to on the engine end. The coolant is definitely not coming from the hose itself, but rather what must be a seal between that metal piece and the engine itself. The coolant level is still at full--it's just a very small seeping. Is this a big deal, and how involved would it be to fix? Thanks...

There is an o-ring sealing the base of the thermostat and the metal housing to the block. This can deteriorate. The block base aluminum can also corrode and pit. Remove the housing, clean it up, fill pits with silicone, and install a new thermo and seal per 700/900 FAQ instructions.

Sounds like your describing where the thermostat is. Not a hard fix. Just soft aluminum, you can twist the stud/nut off easy if not careful. If you are going to remove it, then replace the thermo while your there. WARNING. Many a novice has broken these bolts/studs/nuts.