Here is a rambling collection of my thoughts some maybe relevant most probably not!

The bearings I purchased were from Bolton Bearings and had a 30mm bore (too small for you?) and were about 8 quid each. As I am sure you are aware that bearing is not an ac bearing and probably not a deep groove bearing. Deep groove can take some axial load but a normal bearing is quite limited in this respect. The bearing shown will not have any preload but as you say you could acheive this by pressing the bearing onto the shaft (nut). You will need a fairly close tolerence shaft to control the fit and you will need to haver a fair idea of the clearance in the bearing to work out how much of an interference fit you will need. If you are pressing up to a shoulder than you will not be able to remove the bearing. Mine are a push fit (couple of tenths interference) which can be removed without wrecking the bearing. Compressing externally would give you more control and would allow you dismantle the assy.

The real solution is to try to avoid using a rotating nut, particulary when you are talking about 40mm bore bearings and all the associated pulleys etc that go with it. I expect this is a last resort as you have already looked at other design solutions.

I would be surprised if you are getting anwhere near the design axial loads of a suitable AC or deep groove 40mm bore bearing in your arrangement. The load figures may be based on L10 life rather than ultimate limit, or the bearing you were looking at is not designed for axial loads.

The worst thing for bearing life is not having enough load and the balls sliding rather than rolling and so preload is sometimes used for this reason as much as combatting lash.