That is an LT L83/6l80, totally different animal than an LSx swap. Will cost more to put in and get running, but there is more hp and torque than an LS 5.3.
I think $4200 is high, but that is me, in my area, the same would be about $1500 less or so.
Little more mileage but...24k
https://dallas.craigslist.org/ftw/pt...695586994.html