I would imagine that when the siren found its new home, it was Chrysler-less.
The original 331 Hemi that powered the siren was also quite popular with the drag racers, so as a siren became decommissioned and put out of service, the 331's could be had for pennies on the dollar...or as I imagine "You buy the engine, you take the whole siren" and thus ,the drag racer, sold the rest for scrap. Finding an original 331 Hemi that's not already in an antique car would be practically impossible.
Even the 2nd generation Hemi , the 426 from 1964 -1971 would be next to impossible to find as well that's not already stuffed under the hood of some antique MOPAR. Imagine one of those power plants with the siren!!!
I would imagine that the modern day 3rd generation version Hemi's..... different sized engines since 2003, could be used , but with a certain degree of difficulty due to modern engine technology.
The use of the Chevy engine was probably due to availability of the engine and replacement parts, cost etc. etc. If I ever get my hands on an engine-less Chrysler siren , its going to end up being Ford powered because that's what I know best.