HI If you have a press not a prob'. if not you will need a replacement copper block from a copper hammer or a block of soft all'y, get the nut loose(looks like some one has been in before you with the stilsens/pipe wrench) put the block on the floor and turn the shaft so the gears are at the top and shock the gears off on the block, as in the photo,
If you have a press or can get to one push off the gears from behind 2nd, like this,
to get the nut off if you can get a 41mm socket and turn the end off, then weld on a pipe approx 12-13" long and a old/worn/never used that socket (until you weld it in), in to the end of the pipe, to hold the shaft I have the tool, but you can use a old 5th gear or Rev' gear if you can get hold of one as not to damage the shaft, this way you can torque up the nut correctly on re assembly,
the tools and below holding the shaft,
you can use a old clutch center plate and some pipe wrenches/adjustable, just you do not then know if the nut is tight correct or not , I have pulled box's apart and the pinion and the input-shaft nut are not tight, only nipped or they have been tight and have backed off because the previous had no idea how tight it was and how tight it should be, the gears have then been loose on the shafts, the other thing that gets over looked is that the shafts and EVERY thing on them should be clean and dry on assembly, NO oil any where , hope this bit of usless info' is any good to you .
regards mike