I would agree with Mzungumagic,the 36IV Zenith is the easier carby to refurbush,but they need to be perfectly flat across the base,they also work with an adaptor which turns them 90 deg to that of the Solex, so if you have both the adaptor and the insulation block that would be my choice of unit,BUT, Incisor has just fitted a new Solex to his 59SWB and it is performing very well.
The main thing is that the "O"ring seal in the Zenith is good and that the faces of the 2 halves are flat.
cheers