Chaincase fitting on '36 Model 18

I am in the process of fitting a 1958 ES2 engine to my 1936 Model 18, as the original engine is in need of significant work and I just happened to have the newer engine available to me.

The newer engine goes in fine, but there are of course things that must be changed. I have sourced a set of alternator chaincases off an NOC member, and whilst offering them up a few things have come up.

The main chaincase retaining bolt that fixes to the rear engine plate seems to have seen some considerable action. It's been welded to the engine plate and it seems like the threaded end has been cut and welded back on. The tube could be new, it hardly matters now what happened. What matters is that... not only is the thread at a jaunty angle, which is going to make fixing tricky but also the length seems far too short...about 3/4" at a guess. The original chaincase was able to be fixed by the bolt, but I think it was a struggle.

I've included some pictures of the retaining bolt... should it also have a ring around the bolt behind the thread so that the chaincase can actually be bolted to (or retained in compression against) a fixed location, rather than pushing the cases in the middle against nothing which is what happens at the moment? I've seen a picture of a retaining bolt which has this ring...

and there's more... What is the sealing interface between the gearbox shaft and the floating steel ring on the back of the chaincase inner? The gap is about 1/8" all round. The bronze sleeve floating on the shaft (shown in picture 3) seems to be the main inner bearing bush for the final drive gearbox sprocket. Is a felt seal supposed to run on this too?

I don't think this is all a lack of compatibility between the older frame and the newer chaincase as I can't see how the old chaincase could have sealed either (that's missing it's floating disc altogether by the look of it).

Any input gratefully received.

Images Here