Hei Helge,
I would not worry so much about the torque settings on the nuts, thousands of Triumphs are oil tight with just hand tightening.
There are several other things that can lead to oil leaks at the base;
The condition of the studs are important, the inner 4 larger studs have blind holes, it is important that the studs bottom out on the crankcase, you should see at least 1/2 a turn of clear thread sticking up above the crankcase face. If there is not, then there is not enough thread on the stud. If the stud jambs on the top thread all of the load is on that point near the surface, this could pull the metal up making a ridge that the barrel will sit on instead of being flat on the crankcases.
If you look at the 4 smaller studs, you will see the ones that have their holes drilled all of the way through the crankcase. There is no way for these to bottom out, so they need to be inserted carefully and not very tight. If they are tightened hard, the metal around them will be pulled up. They need to be fixed with ‘loctite’ to hold them in place and also to seal the threads. If you don’t do this oil will work it’s way up the threads and cause an annoying leak. Once loctite is used on the stud, the nut can be tightened normally to pull the barrels down.
If the crankcase face has already been distorted around the studs, removing the studs and carefully countersinking the holes can recover the face without resorting to having them milled.
The dreaded Triumph crankcase step might be causing your leaks.
At some point in Meriden history, Triumph decided that instead of bolting the crankcases together and machining the crankcase face as one unit, they could machine the left and right halves separately and when the bolted them together they would be perfectly matched. I don’t know how they got the idea that they could achieve this on 50-60 year old worn out machinery, but they thought they could.
Sometimes they are flat (lucky) and sometimes there is a 5 -20 thou step between the left and right sides (leaky).
Repair is not easy as you need to set a mill up, measuring from a mandrel placed between the main bearings to rectify this properly.
As Rambo has pointed out, the tappet block seals after 40 years will be brittle and need replacing, this is often overlooked.
You need the special tool and you need to measure carefully that the tappet block is square to the barrel, you will only be able to twist it when you are knocking it in, it will not turn when it is static. Failure to get the tappet block square means the valve timing will be unbalanced between the two cylinders.
Good quality O rings for the pushrod tubes are needed, a variety of thicknesses is useful to help get the correct pressure on them.
I might have made it sound a bit daunting, but it is not really.
As for torque settings, just tighten evenly by hand, run the bike, let it cool down and check again.
Regards
Peg.