Well what will fit doesn't have anything to do with being 20s or not as long as the rolling diameter is about the same. Its all about what offset they are, and how wide. If a wheel hits the strut, changing the offset moving the wheel out will allow it to clear the strut. If a wheel rubs on the guard changing the offset to move the wheel inwards will stop it. How far out, or how far in depends on the overall width you want if that makes sense.
I have 10s on the back of my VR with IRS and they do not scrub at all. But I had them made to my own offset so there is the same amount of clearance on the inside to the trailing arm, and the same to the edge of the guard. As above live axle is a little different, principal is the same though.
Steel rims can be made relatively easily to any offset, I have in the past had rims changed, they grind off the welds holding the centre to the rim, then move the centre to the new position and reweld it. In fact I am about to have a pair of 15x10 steelies made up for drag use.
Hope that helps.