No way in hell does Buchanan do that well. he'd be extremely lucky to even do as "Well" as H.W did.
Considering that he manages to beat an incumbent Republican president in the primary and gets the endorsement of Perot (who probably campaigns for him as well) in this scenario, I find it extremely unlikely that he would lose in a massive landslide. I also disagree with the assertion that populism was terribly unpopular in 1992.
Populism was unpopular in 1992
Bush Sr ran on a Reagan's 4th term
Clinton ran on ending welfare as we know it , tough on crime , opposing tax and spend policies and reganomics lite
Perot ran on balencing the budget
All of those policies can be populist. Populism is a type of rhetoric, not a set of policies.