1970 318 duster exhaust question???

There is such a thing as back pressure. It's called a restrictive exhaust. LMAO
It's more in the way people think and the went understanding of the terms used.

The slower velocity of a to big of a exhaust pipe will slow a car down. The high velocity of a smaller pipe will work. But the problem most people have is with the wrong size for the combonation.

For a stock 318, a good size is 2-1/4 for dual exhaust or a single 3 inch to maximize everything. Going larger can be hurtful. I did do a 2-1/2 many many moons ago. It just seemed loud. Not fast. 2 inch seems small t me even for a sock engine.