When you move the axle to under the spring you have to leave the axle in the same aspect, by that I mean the top and front of the axle stay the same, if not then your alighnment and brakes are out of whack, you will need a new spacer on top then sit the spring on that and the fish plate is positioned on top of the springs, it is always advisable to use new U bolts, nuts and fish plate.
When positioning the new spacer you will have to measure the exact position of the old one and place the new one in the new position in the exact same position ( like mirror reverse ) that way you should keep the axle alighned, now before you pull everything apart you should measure and mark on the chassis 2 datum marks from set points on the axle then when you move everything and it's all set up you use your datums marks to make sure you are in the right position, when it's right you then weld the spacers onto the axle ( just a good tack ) and finish tightening the U bolts.
You can get new spacers when you get new U bolts.
If you do this job yourself you MUST have 4 x Jack stands and a good trolley jack, 2 tonners all round are good, you will have to lift your rig up a good way to make it easier and drop the front spring hangar, good idea to check and replace pins and shackles as well, wheels and hubs removed make life easier also.
PS: this will usually raise your rig 80 to 100mm and change the height of the hitch on your vehicle as well, check that your hitch will give you that extra height first. Legally tow hitch has a max of 490mm ?????