I couldn't say when they made the change. Sometime before they went to the newer style hood without louvers is all I know.
To go to the lower tin you would need a different cowel around the tank and the front bracket that bolts to the head. On a gasser I think the firewall tins are adjustable heightwise.
On a diesel there is a bracket that would need to be changed.
The bracket that the lower part of the nose bolts to has bolt holes for both heights so that one doesn't need changing.
Notice the different placement of the hole for the steering column in the photo below.
The yellow cowel is from a high tin tractor.
Rather than change all the necessary parts out I think your idea of welding a taller fill neck on to the new tank is the best one.