A builder after my own heart - I too like to get it straight from the horse's mouth so that I sleep well at night.
Look in the maintenance manual heavy applicable to the 912ULS in chapter 71-00-00 section 6.1.3 p.20, where is says '6.1.3 Engine suspension frame assembly
See Figs. 71-10 and 71-11.
Screw the engine suspension frame (1) to the engine housing with the
lock washers and the allen screw M10x110 (3) and M10x35 (4),
tightening to 40 Nm (29.5 ft.lb) in each case.'
I installed mine just two days ago and had a **** of a battle for three hours getting the frame over the lower left water pump tube. I had to eventually accept that the powder coating had to be damaged to get it on. The thickness of the powder coating was the tolerance. I touched it up with a dab or two of 2K paint.
You will probably find that a regular torque wrench and allen socket will not fit into the M10 Allen screw above the number four cylinder. I cut a 25 mm length of 8 mm allen key, which I inserted into the M10 cap screw. I then slipped an 8 mm ring spanner over the short length of allen key. I tried torquing the open end of the spanner (compensating for the extra lever arm) but the open end was not up to it and bent open at about 30 Nm (equivalent to about 35 Nm at the M10 cap screw), so I then rigged up a tight fitting pipe over the spanner and suspended a carefully calculated weight at the end of it to reach 40 Nm. Again, I stress that this is what I did, and by no means should this be construed as advice on how you should do it.