Well it sounds like the the only reasonable, responsible course of action is to replicate the original test but use modern high speed recording equipment to determine what would happen to that steel plate.
How critical is the nuclear explosion in this experiment? Could you use an equivalent traditional explosive? 1KT explosion can be achieved without going nuclear.
According to the wiki page the Pascal-B test was the equivalent of 300 tons of TNT.
hold my beer
I'm not a physicist but as far as I know a nuclear explosion releases its energy much more quickly than e.g. TNT, so just releasing the same number of joules wouldn't exactly duplicate the test unless you were able to release them in the same number of nanoseconds.