Most older wood lathes have around 36″ max between centers. Anyone who has made chairs knows that this is a limiting factor. Most larger rocking chairs have 40″-42″ back post. I have made do with some pretty stupid and unsafe rigs to turn longer posts but finally decided on a more permanent solution for the short tool issue.
The lathe I modified here is a 1970’s era Delta picked up from Craigslist a couple years back for $100. It had bushing problem on the Reeves drive and a three phase motor, both were easy fixes. This lathe had exactly 37″ between centers before modification.
The extension was made in place on the lathe to assure alignment and fit and would recommend you do the same. The lathe bed is cast iron and brittle, you would not want any undue pressure from the mounting bolts created by a bad fit.
I know most woodworkers don not seem to venture too far in metal work (or vise-versa for that matter). If you have an older lathe and wanted something like this made, any metal fab shop could make one.
How long could you make one of these? The one above extends the bed 10″; I would be uncomfortable with anything over 12″. The longer the extension the more pressure exerted on the cast portion of the bed. Of course, if you are in the business of making those terrifying YouTube videos turning a tree trunk, by all means, make it longer!!