I did just that with the 54 blade I got for my 420. It was originally a 400 mount that I removed the studs and had a solid bar welded in with a 3/8x6 inch gusset welded in the center of the bar and mount.David,
The plow mount in the second pic appears to have been built originally for a JD 400, assuming the frame spacing is about 16". The lower mounting studs were cut off or additional holes were drilled and the rod was inserted. The plow should mount on a JD 420, 430, or 400. However, without some inward support (as offered by the 420 plow brackets), the rod will bend when pushing some good loads. Welding in some inner brackets will improve matters.