The only difference with mk1 guards is on the sedan(4dr) which has bolt on doors , this is a minor fix with some light trimming to the inner strip which is spot welded to the A post.
Mk2 van guards only fit mk2 van and the van is a lazy ford follow on model with a few front end Updates to match the sedan. . As for RSing it , you keep the mk2 van guards and cut back a notch to take the nose cone. Once it fits up yu would be best to trim the nose cone where the style line is and adjust to match up with the mk1 style line.. Some plastic welding and is ready to go.
Personally I'm not a fan of RS snouts on vans... Keep it flat nose you'll save time and about 50extra kgs !!
Back in the '90s when RS gear was crazy money for parts, I converted a genuine RS 2dr to flat front. .. mind you that was when a mk2 costs a few hundred bucks and the nose cone was from $500 !!!