Leaded,
I wouldn't worry about backpressure.
I've not seen any data (nor my own) that shows backpressure improving anything anywhere.
Sometimes people hook the mufflers up, resulting in backpressure, and the car goes quicker. Why? I propose two reasons:
1. The engine was not happy with open pipes because it was jetted too lean. With open pipes properly jetted it will go faster than thru the muffs properly jetted.
2. The pipes added length - the header length (primary+collector) was too short, and adding that length of pipe to the muffler made the engine happier, especially down low. Same effect could be had by adding the right pipe length but with no muffler on the end.
You might add true long-tube headers for more low end, or an H or X-pipe to your 2-1/4 exhausts to create a pressure pulse at the right spot. All the way to the back of the car is probably too far unless your peak torq is at 1200rpm or so.
Yes a 4bbl carb would add low end since you can use some very small primary barrels for low end torq and then have the secondaries kick in so you don't lose top end. Holley 390, or Edel/Weber/Carter 500cfm (fat high response primary boosters) would be good choices for the most low end.
