There just was a thread on the visual & other differences between the OE/aftermarket holleys several weeks ago iirc. I'm not sure how to search for that. For an aftermarket Holley I'd want one of the clones that has adjustable systems that the OE ones did not have. Not sure if a person could get an OE one & rebuild it & use an aftermarket primary metering block with those adjustable air bleeds etc. I'm not into Holleys very much so my help/info is limited but here's a BTT for ya