JAF GT300, though, that's where some crazy shit happens.
That class lets you get away with a *lot*. As long as you've got the firewall and A-pillars from the car you based it on, it's somewhat similarly sized to it, and it's got an engine from the same manufacturer... go nuts.
There's been a Daytona Prototype car that was converted to a road car so it was eligible for GT300, then converted back to a race car (the Mooncraft Shiden).
There's a Subaru BRZ that runs a rally car engine.