Interchangeability of overdrive types on gearbox?

[Paudman]

I have an overdrive gearbox which I'm going to use on a Herald restoration. It's originally a Spitfire
box with a (I think) D type overdrive. (GR box number but below it very lightly stamped is FK... needs a good clean to make it out properly.) As I much prefer the simplicity of the J type, can I simply unbolt the old overdrive and replace with a reconditioned J type?

[Doug Paterson]

I'm 99% sure you can't.  However I've been wrong before.

[Clive]

but not this time!

[CharlieB]

What if you get one of canleys adaptors, or is the mainshaft different?
http://www.canleyclassics.com/products.asp?article=overdriveadapterplate.xml

[hypoid53]

You require the correct type of mainshaft and adapter plate for either the D type or J type. They are not interchangeable. You will also require the mounting plate and rubber for the rear fixing between chassis rails.
Here is a J type on a GT6 3 rail box to give you some idea.