The Volvo "hunchback" (that way we nickname that car in my country) can be combined with a late B20 Volvo Engine. This motor is easy to push up to 180 HP. But it's still a inline four.

A common conversion is a Chevy Small Block (to heavy on the front axle) or the Rover 3.5 litre V8 which is similar to the 48 Buick engine. This motor is full aluminum but heavier than the B20. 250 HP with no problems.

The engine of the Simca Vedette mentioned in this thread is the Ford Flathead V60 with Ardun Heads. Nice set up but expensive.