Hp per gal. Is what I call fuel economy, but my ram is bone stock, well ok not really most of the exhaust fell off, (aka removed by me) and I'm running a diablo, but I don't call it modified. My duster though, will be far from stock. It has a dana60 out of a 69 F250 under it, modern built Hemi, and I'm shooting to make 4digit power to the wheels. I'll be happy if I get 2 gal per 1/4 mile.
But the op has a daily driven street car and economy matters. I would be bummed to get 8-10 out of the swap for a street setup. But 14-16 is great.