Replacement 1/4 rear chassis

Assume this is a dumb question - need a new 1/4 rear chassis for a 2003 D2 - found one which has 900mm extensions rather than 600mm - does that just mean the “arms” are longer and therefore can replace more of the rusty bits?!

Yes.
Just be sure that 900mm is enough !
I ended up going with a 1400 mm section & was glad I did when we cut off the rusty section which was remarkably dire even around the damper mountings which you'd imagine were a "high point" on the chassis & therefore not so prone to water staying inside rotting the steel away !

Thank you!