Once upon a time, rubjonny said....
it all depends what size engine and the age of the cars in question. If its a 1.0-1.3 then you cant fit a 5 speed gearbox directly, as there are no MK1 1.0-1,3 5 speed gearbox mounts in existance
What you do is weld half a MK2/3 1.0-1.3 5 speed Polo gearbox mount to half a MK1 Golf 1.0-1.3 gearbox mount
If its a 1.5+ engine then any MK2 5 speed box will fit, the mounting points you need for the MK1 mounts are there. But some early MK1 1.5+ gearboxes wont fit MK2s because the mounting points needed for the MK2 brackets are missing.