The big problem with torsion bars is you can only go so big before you run out of choices. With a 400# wheel rate, the biggest torsion bar out there, the small block will outhandle the big block for longer on a stock suspension set up.
Since the XV Level 2 kit uses coil overs on all four corners, you can get any spring rate you want to support the bigger engine. Bigger spring rates equal better support for those bigger engines. Go big block if your willing to step up the spring rates.
Or, like greenfish said, go late model Hemi, get big power and light weight. Just be prepared to pay.