Firstly, the hour-glass shaped piece is not symmetrical top to bottom- it is meant to go a certain way up, so that there is no gap underneath it ( long side on the bottom ). This picture shows the issue:
Make sure you are installing that piece ( top right ) the correct way up.
As for spur gears, you will either need to notch the chassis / braces to fit the 58t spur, or you will need to switch to smaller MOD1.0 gears which are more common with this set up ( same as most 1/8 buggies and truggies use on their centre diff units ). 48T tends to be about as large as you can fit, though more like ~44-46T maybe needed.
For centre drive shafts, it depends exactly on how you position the CD unit- most guys cannibalize shafts / dog bones & drive cups from other models which are the needed length. I'd look at 1/8 buggies & truggies, and most MTs like the savage and LST etc too- they tend to list the shaft lengths ( pin to pin, or end to end, easy enough to work out what lengths you need once you position for CD unit ).
There are several threads around ( search 'emaxx slipperential' or 'emaxx center diff' ) though details about dog bones used is sketchy- Mike can custom make them to order ( or used to ), or you can go with slider shafts made from stock plastic shafts with an extra yolk on one end.
As the item description says though, it does require some DIY so there won't be an out-of-the-box solution to mounting & shafts etc...