RM400C swingarms are made of aluminium, not steel, but look a lot different to the N & T model swingarms. Most guys that fit Suzuki swingarms to XT/TT 500 Yamahas use PE 250/400 swingarms, I think, because they are wider and go close to fitting straight in. But I am not sure what would be best to suit a Honda XL500. Maybe a KLX 250 swingarm would be worth measuring.