David, i had to do mine aswell, what i did was get the replacements, and then look at the job thoroughly, what i found was that the new ones appeared to be the right size to be placed inside the old ones through the chassis, so, i cut off the old ones at the outer edge of the chassis, then cleaned up the inside of what was left and basically hammered the new inside the old, you clearly need to keep the 'flange' bit upright..... cos you won;t twist it once its in the chassis..!!
I then welded around the outside, and called it quits........ i am however now changing the chassis, i only did the outriggers as a short term job, hence i wasn't maybe as fussy as if i wanted it to last for years and years.......
Hope that helps.
Ron.