size & "H-pipe"
OMM, your example reinforces my point to the original poster (who has now disappeared or run for cover). The biggest determinants in figuring best pipe size, length, and cross-connection are engine displacement and rpm range. The OP wrote that he had a "Mild street 440 4bbl" from which we could guess that powerband of interest 3500 to 5500. Maybe even lower if never racing.
velocity is important for primary tubes on the headers,collector sizing is also important but after the correct collector lenght has been achieved all you want is minimum restriction.
On this I have to agree with Lone Wolf. Here's why. First, in a full exhaust system, the 'collector' length becomes the distance from the primary pipes to the first interuption - be it an H, an X, a muffler, or a resonance box. Second, as the exhaust temperature drops going from the primaries to tail pipe, the exhaust gas density increases and takes up less volume. From these two points, lets look at the question of achieving the highest flow possible at all rpms. (All because we're looking street or street/race situations).
Agreed that the maximum flow is achieved with the least resistance in the exhaust system. Therefore agreed that a restrictive system increases pressure for a given volume per minute. In other words, a restrictive system hurts flow. At the same time, allowing any point inside the exahust system to reach atmospheric pressure will cause the exhaust flow to stall inside the pipes. This actually happens on some race oriented vehicles (especially motorcycles) and causes problems for low rpm readings on WBO2 in the tailpipes. In many cases its an easily accepted price for more power where it counts.
In a long exhaust system, the diameter can be reduced slightly getting closer to the exit for purposes of maintaining mass flow and velocity. If the exhaust gas temperature dropped about 300 F, about a 1/4" diameter smaller could be used at the tail pipe to maintian velocity. Think of the tailpipe as a nozzle and the objective is to keep the mass flow of the exhaust moving beyond the tip.
I don't know if 300 F is a fair estimate, although I do intend to take some measurements if given a chance. My Innovate datalogger accepts K type thermocouple, but its one more thing on my to do list.... I think the temperature drop will be rpm dependent, with more cooling at lower flow.
