I am a mere novice
in the boating
world , however my specialist field is the manufacture of special purpose machinery and machine tools .I do this on a daily basis and from the sharp end not from a chair .
At a glance it would seem that you must either ditch the tank or take steps to provide a properly engineered drive solution .
By this I mean a substantial shaft ,the diameter of which would be dependent on the second part of the job which would be to cut out the tank tube and replace it with a heavy duty structural tube carrying a bearing at each end and one or 2 in the centre .
This tube would of necessity involve a lot of machining work
to provide the necessary bearing seats and line bored accuracy and would require the tank to be capable of providing support for the tube ,by means of welded in end fittings .
Then having achieved a true running structuraly sound bearing tube and shaft ,which in effect is now part of the tank , provision must be made to ensure the engine
is coupled by some sort of cv type joint ,in order to ensure no engine movement is transferred to the tank/bearing tube.
It should be possible with careful work
to align the propeller
end of the assembly with the cutlass bearing .
I realise this is probably not what you want to hear , but I every week I see half done badly engineered work that costs thousands to repair and the best way to avoid heartache is to do the job properly .
There are many wonderful products available in the machine tool world for replicating shafts and bearings into place using resins etc , but these will be of little use in this case .
I wish you the best of luck with finding a lasting solution to this problem