Yes Poncho thats what I was trying to say. With a a smaller displacement 305 or even a mild to mid 350 you do not want too much free flowing exhaust because all your performance gains will be too high and your bottom end will fall on it`s face. Now if you put together a 350 that has a huge cam and a high end intake then I could see a more free flowing exhaust. But with a 305....I would go with 1 5/8 "Shortie" headers and put on a Y-pipe. The 305 has it`s power on the low end...Why take away the power range? The 305`s power range is with the lower rpm`s ...if you take that away that 305 would be a dog.
Now if you don`t believe what I`m saying...try this.......( here`s a in-expensive test for you ) Put your headers on and then put on a couple of cheap flex pipes with the mufflers you were going to put on....just wire it up and Drive the car and you will see it won`t have any acceleration.
I have experimented with the 305 and 350 with a few different combos. I haven`t found a dual set up worth while for a "Street" set up. BUT...if you toss on your headers with some small tubes to give it some back pressure to help out the low end.
Like Luci touched on ...there are charts out there that tell you where your performance is. December issue of Car Craft touches on headers and it gives a chart, the bigger headers gives more higher end performance. It`s the same thing as your exhaust pipes.
I`m sure that 305 is not running at 4500 + rpm`s on a regular basis.