Others have provided good advice here. I have D type in my 1800 and performed only simple maintenance on it when I had it on the bench recently. In addition to the O ring replacement mentioned, it can't hurt to replace the gasket behind the strainer plate. They are still available, as is the strainer. In my case, I also replaced the wire from the solenoid to the switch when it was easy to use solder and shrink wrap. My mechanic instructed me to use only 30 wt. oil in trannys with OD.
Are you comfortable with the corrosion on the thermostat housing? New housings are less than 50.00 USD and, in my view, cheap insurance.