You may have two issues. If you are sure are not actually overheating with your thermometer readings 70 something at the top of rad and lower at the bottom hose is good. With engine hot and rad cap off [careful opening up, slow with thick cloth over cap] can you see coolant circulating? If so...

Petrol series known to cut out due to fuel vapour lock because of the overheating. Are you losing coolant? Has your rad got all of its cooling fins? A hand held IR temp reader is a good tool to check which parts of a cooling system is doing its stuff.

Brush pack and voltage regulator one combined on many alternators. Unless you have some equipment on board with a heavy drain and in use for long periods on the batt there is little point in having a larger than standard alt. All it will do is charge the battery faster while loading the engine more.

Double check connections ? With alternators the usual suspects are brushes and their contact rings, voltage regulator, and rectifier. How to check/repair alternators on you tube. This does mean removal and opening it up.

New belt and re tension after a run. Correct width belt? I have found solid belts better than those teeth type.

Have a look at dieselinjectionpumps.co.uk Parts and information/diagrams available. I think that cover is for the pump advance mechanism, a spring/plunger is in there. Cover has a gasket or O ring to seal. I am no expert on diesel pumps but parts/info from this firm has helped me repair both...

Your Landy is not a Defender [ the name came later on] Yours is a Land Rover One Ten, this detail can make a difference when ordering parts. Looks like a Lucas 127 type alternator.

Looking pretty good to me. What engine? From the TDI decal on the wing a 200tdi? Common enough change for early Ninety [written not numbers, can make a difference when looking for parts] Mine is a 1990 Ninety with 200tdi.

There are spot weld removal bits available, better than a standard bit.

I would put a spanner on that union with the rubber pipe and see if it will nip up a tad. As you say oil may be coming from above but that is the diesel injector pump. Air flow when driving can sling oil all over.

I don't know if any help but Bahama beige was a popular colour at that time, mostly on Range Rovers.

When Land Rover brought out the Series 3 they started the suffix at A again. But now just to cause us confusion the location is on the top edge of the transfer box. Then there is the issue of what other folk have done in the past with fitting parts and cases from different eras.

One of mine went to leak recently, no issue with the ball housing but found a bit of water had got into the back of the seal and rotted the seal spring until it gave way.

That is the way I do it if other parts in good order. When out I lower the hub swivel unit back into the wheel laid flat on the ground, holds it nice for seal change. Extra hands to get them back is good, more so the long shaft.

Off with the dash top to access the wheel box's. Swapping out the cable at motor end bit of a fiddle but several how two video's on the you tube.

It doe's sound like there is a lot of "slop" in the system. Can you move arm by hand to full sweep and usual park position? If so there can be issues with location of arm on its spindle or with the drive cable itself. Also can be problem with location of drive cable with the motor gear box.

What model is your Land Rover? There are several different Series box and each has a different Suffix number/letter. Gears look different to me and so will not do. You could swap the whole transfer box section.

Moghrey mie and Bore da, From the West Wales coast. I listen to Manx radio a fair bit as it comes in well hear and has good music. Try carpricetracker.net May help with historical prices. Hwyl.

In many years gone by I did use the 36IV for other uses other than my Land Rover. I had one on my 1300 Escort. Experiments with different jets had me a good cheap power up grade [along with other tuning] over the original set up.

It may be worth trying a re bleed.