My 512 uses short headers, but they are 2-1/8 to 2-1/4 step headers to 4" collectors....I would not even consider anything below 1-7/8 tube size.
Mopar BB large C.I. motors don't like small pipes, especially on the exhaust side of the collectors
I saw the difference on my old BB 1969 Dart GTS at LACR years ago, it had a set of Hooker 1/78 primary pipe with 3.5 inch collectors, It had a set of exhaust pipes and mufflers that were 2.5 inch from the collectors into s set of Thrust Corvair Turbo mufflers 18 inches back from the collector and then around 20 inches out from the mufflers to the edge of the car in front of the rear tires. The car ran 3 runs in the 12.31 to 12.23ET at 109.4 to 109.7 MPH by using different RPM shifts with that exhaust system on.
I remove it and the next three runs varied from 11.39 to 11.41 ET at 117.5 + mph
My pump gas Duster had 3.0 inch exhaust system from the same Hooker fenderwell headers with a set of Magnaflow 3.0 inch straight threw mufflers mounted at the rear bumper, that car didn't vary .02 ET and less than .02 MPH in the 1/8 mile by removing that exhaust system
Use 3.0 inch pipes from the headers back to the end of your system
