A slant 6 is not enough power, and a 440 is too much. There is a LOT in the middle. If you REALLY want a lot of streetable power, a 408 as suggested by
@jazak5 would be hard to beat. Next step up would be a stock 318. That would give decent mileage and decent performance. Or you could easily warm up the 318 with a mild cam, good intake, headers and dual exhaust. That would give good performance and decent mileage. The next step up is a warmed over 360. They are easy to find, and 400 (+) HP is easy to get. Performance will be up, and mileage will be down. If you want to drive it a lot on "long trips", a fairly stock 318 with highway gears would be great, but it may not have the performance you want. The decision is yours. Only you can decide what level of performance you want, and what level of drivability, noise and mileage you want. So what do you want?????