Swap in a 5.3L LS4 from a late-model Monte Carlo SS/Impala SS/Grand Prix GXP. They're already transverse and their accessories are set up that way. All-aluminum, 300ish hp with modern computer controls. Easy to program the computer systems with LS Edit and there are a bunch of skilled tuners out there to get it dialed in.
You can buy a complete drop-out for $1500-2500. Here's one in California:
2006 Monte Carlo SS 5.3 Liftout Engine Complete 24k LS4 | eBay