Can you fit 26 inch rims on a Chevy G20 van?

No, you cannot fit 26-inch rims on a Chevy G20 van without significant modifications, and those modifications would likely be impractical and unsafe. The G20's suspension and wheel wells are not designed for such large diameter wheels. You'd need to heavily modify or replace many components, including potentially the suspension, fenders, and possibly even the frame. The resulting vehicle would likely handle poorly and be unreliable.