Keep the cavy stock and reliable enough to drive on a daily basis. A stock cavy that starts and runs every time you need it beats walking any day.
Save that money to build the Charger. The Charger will be a LOT more fun to drive than the cavy if you're looking for performance. I remember the Charger when it was brand new. With a good motor, they're a blast.
If your friends are so hopped up to build the cavy, let THEM pay for it.
I agree with millrtime. I like the old American iron.