Joe and Tony know what they're talking about as they have done it before. It's a contortionist's job and if you want to show off just how flexible you are then leave the engine-lid on and save yourself five minutes up front to lose half an hour in doing the job.
If your car has been properly restored then you will have an under-tray, which is why there's no point jacking it up; in any case the air duct inlet is angled in such a way that you need to get at the nut from above and behind the engine.
I would find it a bit of a challenge and if I had the problem I would take out the engine as suggested by Tom.
Even with the engine out of the car it's hard to tighten the nut to a satisfactory torque because there's nothing decent to get a grip on to stop the dynamo spindle from turning; there will originally have been a factory tool for that.
I use a new nyloc nut with Loctite on the threads and secure it using a pneumatic impact wrench...not something I could do in situ.
I can get a socket on an extension bar on the end of the fan spindle through the air intake housing and with my right hand and a spanner on the pulley end with my left hand to tightened it up. As it’s a nyloc, as you say last time I applied a bit of loctite, it should stay nice and tight.
Admittedly I am pretty wedged in there with my head tucked in behind the air filter housing but it is possible if you are small and bendy enough!!