Are Shimano road freehubs fundamentally different to mtb?

I just tried to replace the knackered old freehub on an Ultegra 6500 series rear hub with a Deore one I had lying around. The first inkling I had that it might not work was when I saw the loose bearings in the Ultegra one after removing the axle - the Deore on is sealed, like the MTB ones generally are.

I was able to fit the freehub to the hub shell, but when I tried to put the axle back in it got tricky. The cone sits much further inboard than it did previously and I can't for the life of me set the cones so that they spin freely.

I suppose this is normal since the cone is looking for bearings that aren't the, but is there any think I can do to work around it? I'd rather have the sealed freehub on there for longevity. Can I plunder some parts from an old mtb hub, I wonder?

Cheers,
Gareth.

they share the same DNA - I usually use the road axle and mix it up with the MTB cones and see what goes together.

Strip both right down to their component parts, then you'll see what fits.

I've often built up an MTB axled/ freehubbed road hub with all the seals of MTB for the road.

It normally helps if you match the freehub body and the drive-side cone. Sealing arrangements can be incompatible betwen different models. If I have a donor hub to hand, I normally transfer the entire drive-side stack (cone, locknut, seals and spacers) along with the freehub body to the receiving hub. If I need to make any adjustments to the overall width, it happens at the other end.