Air cooled Porsche flat-six. You could hang it behind the rear wheels in classic 911 style, or go mid-engine for better dynamics, as in the 1970-72 Porsche 914-6.

Or, find a candidate car with body-on-frame design, such as a late seventies GM midsize, or even a Chevy S-10 pickup, and use that as a foundation for the body.

Or, build your own old-school kit car, using a old VW Beetle floorpan.

If you plan a very tapered front end, not having an engine to work around is helpful.