What I meant by the "axle thing" was that the castor is axle dependent and not vehicle dependent. The correct castor for any specific axle is the castor that was set at the factory. If you change the factory castor of the axle, you change steering input geometry (and it doesn't take much). So, it really doesn't matter what vehcile it is going on; the factory castor for the recipient vehicle shouldn't really have any bearing on the setup for the donor axle on the new vehicle.
That being said, don't forget about differences in suspension geometery. If your spring perches are angled differently (between the donor vehicle and the new vehicle), you will want to take that into consideration. The best way to do this is to bolt the new axle under the vehicle and set the correct castor at the knuckles before welding them up.
Anyway, I don't claim to be any 'expert', but every time I have seen someone change the factory castor on an axle they seem to complain about poor steering, death wobble, etc.
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