If you're serious I'd look for a early to late 90's GM 6.2L or 6.5L diesel from a light duty pick-up. Physically the engine is between a standard small and big block Chevy, stock mounts should work as well since it was the upgrade engine over the basic gas motors.

The 6.2s were normally aspirated, the 6.5s are most commonly found to be the turbocharged variety. These are high compression diesels, 21:1, with boost limited to around 6lbs or so. The motor is still in production, it's what you get in a real Hummer.

The 6.2s were around 150bhp and the 6.5Ts were right at 200BHP stock. In marine trim these were bumped to 300-350BHP with somewhere near 650ft/lbs of torque. Different pistons brought the compression down to 16:1, then boost was raised to 20lbs.

I'd find a rotted out pick-up and transplant the entire drivetrain. Take the hydro-boost pump for the power brakes (assuming the Chevelle has PB).

Stay away from the mid 80's GM 5.7L diesels as these were a POS.

Regards, Mark