The easiest way is to just get setup rims. They just have a deeper drop center to fit the 34" casting and then the rim diameter is the correct one for the 38" tires. You use your 34" casting and wedges.
They do make step up wedges if you already have the 38" tires and wheels. I know they did make them for JD 4020s. I have used them before.
I have attached a picture of the stepup rims and the step up wedges. These are Unverferth ones.