June '87 325is 200k+ miles

I have coolant leaking from the hole just above where the main driveshaft connects to the transmission. Seems the hole is a vent or overflow release for cabin heating/ cooling system, but not sure. Coolant really starts dripping out once the first white tick mark after blue area on temp. gauge. A bit of coolant and fumes came into the cabin from the vents when I turned on the heat when I first noticed car getting hot. Car starts to get hot, past middle white mark in about 5-10 miles.There is also a little sputtering of coolant from the muffler/ cat connection, my mechanic had to weld the end cat tube flange a year ago, and this may have become a leak area now. I thought there was some coolant dripping from the front area of car, but not sure.

I have a cracked header downpipe at the flange weld, for over a year now, which may contribute to or be the problem. I get a rattling noise often when driving as a result. I smell some gas when hood is up and engine initially is on, and seems like a puttering noise. The engine idles ok, doesn't jump in revs, and otherwise drives just fine.

Before this coolant problem my mechanic suggested he remove and weld the header downpipe to take care of the rattling noise. Any suggestions please how I can fix coolant problem, what best approach? Thank you.