A way you can go when setting up a bearing is to have it mounted on a plate, with slots or oversized holes for adjustment of the flanged bearing.
Although my own personal choice would be to have 2 bearings, and universal joints and a short drive shaft to drive the actual prop shaft, but whether you have one or two, this setup will work.
The flanges are normally square. You position metal blocks welded on just clear of each of the 4 faces. They are drilled and tapped, and have bolts through them that can be adjusted to butt up against the bearing.
To set up a single bearing, you take a dial indicator, and place it in the vertical position. Set it to zero. Pull the shaft up to it's fullest top position, finding the normal position it would be in if the boat was upside down, that is, do not pull beyond a reasonable resting place. Take a reading of the dial gauge. Say it is 50 thousandths, then you adjust the screws in the appropriate block, in this case the bottom one, and raise the shaft/bearing up by 25 thousandths, then repeat the process for sideways play, pulling to one side, (like gravity did when doing the vertical one) zero out the dial, then pull the other way take a reading, set it up by splitting the difference of the two readings. then tighten the main holding bolts.
You can still do this without the blocks and screws, but they make it easier. They are also a way to maintain the bearing position if the main attachment bolts work loose, buying a bit of time. If you prefer not to have them, try pinching up the flange bolts hand tight, then tapping the bearing flange sides to move it in small increments once you have the readings.
A far better way is to have 2 dial indicators. Then you can be sure that one axis does not move when you are setting up the other. But you can manage with one if care is taken.
The idea behind this is that the shaft itself ends up in the centre of all the surrounding clearances that are currently there.
This info comes from experience as a machinist/millwright and may have snags due to the marine application, I have only just got into boats, so other group members might wish to correct errors/opinions.
My own personal preference will be to separate the prop shaft, and have the drive through a short drive shaft. I am assuming that the prop shaft has two bearings, one each end of the tube. To have 3 of 4 total on the same shaft is mechanically unsound from a practical sense, particularly if the mountings are likely to flex, such as with a boat hull.
If that is the way you choose, you might be better with two plates to support the motor(s) and bearings, to enable side load without impacting the prop shaft.
Maybe also take a look at fabric universal joints such as are sometimes used in automotive steering on some older vehicles, they are forgiving, and would probably reduce vibration, and require less maintenance than regular UJ's, although of course there would be power limitations, however, they were used for the main drive on pre-war Fiat Topolino cars, so that would indicate a capability of at least 20HP.
I would also favour the motor(s) being mounted from behind, allowing easy access to the drive belts.
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