What engine for my dart?

What did the car have originally? Add a rear end swap to your budget. Was it originally a 4 speed car? How big is your money pile?

You can run low 13's or high 12's just fine with a moderate 360. And it fits the cheapest and easiest.

If it were me I would do something like KB 107 pistons, EQ heads, performer RPM intake, XE274 cam, 750 carb of choice, headers and bolt it in. Should get you where you want with the right gears.

Then again, you have most of the parts to build a 451. But unless you plan aluminum heads, intake, water pump, and housing, it will be heavier than the 360. And then you spend more for headers, transmission cases, etc...