Headers and collector length

I've been reading a lot of technical information regarding header sizing and it's effect on engine performance. I understand the relationship between primary diameter and length, as it pertains to V8 engines. Lately, I've been focusing my attention on collector specifications, particularly collector length and it's effect in a full exhaust system. What is difficult to find an answer to is at what point the collector no longer a collector. If for example, my headers have a 3" diameter pipe measuring 6" long (from the tips of the primaries to the gasket flange) and I run 3" exhaust to the rear of the vehicle and then into low restriction mufflers, have I effectively increased the collector length?  There's lots of tech talk about putting mufflers at the end of the collectors (suitably spec'd mufflers) and calculating length based on that addition, however for a daily driven street car that sees drag strip duty, a full exhaust is desirable. In my application, the mufflers are behind the rear axle.
Any insights?