trailing arm compatability???

[defgone]

i have a 63' C20 long fleetside....will standard c10 trailing arm fit???....i am putting a 76 c10 suburban rear axle on so there is already some work being and it will mostly be a bagged c10 when i'm done...took it apart today and my trailing arms are rotted bad...i can find c10 arms but nothing specifies if c10 or c20 are compatible...also is there a difference between short box and long boxes??...thx in advance for the help
Tony