360 build advice for my '70 dart swinger

It won't be easy to find a happy compromise between performance and mileage. Dual 4 barrels migh look neat and can be fast, they are not easy to tune. In many cases, a properly tuned 4 barrel on a good aluminum hi-rise intake will outperform a low-rise dual quad set up. Having too much airflow is just like too much cam or big ported heads, it will hurt low end and midrange performance, which is where the engine will spend most of its time, not to mention fuel mileage.