I feel your "pain"
Having dealt with several of these companies for axle hard lines for the various rear disc brake kits I've created over the past number of years, I can attest to the confusion that seems to prevail. Frustrating, yes, but as long as all involved keep their cool, it is possible to eventually get what you need.
What I do not know is whether any of these automotive hard plumbing suppliers have actually made products for the OEM's, using production drawings, or if they're totally based on copying sample lines from individual (private) customers.
As far as I know, the online catalogs for Classic & Inline are, in the main, based on what they've been asked to produce, mainly by restoration customers. To my knowledge, these companies do not bring in complete vehicles to pull individual plumbing items and copy them--it's based on parts sent by those needing a replacement for "this or that" which the suppliers can justify producing, cataloging, etc.
The bread & butter of their operations reside in cars from prior to 1980, an era of vehicles which has been thoroughly massaged and researched, while the late B-body, as much as we view it in the same way as those older vehicles, hardly qualifies as something that is going to produce anywhere near the business for them as the cars being restored by muscle car and/or pre-smog (before 1975) era enthusiasts.
It may be that some day, the year by year configuration differences for the B-body will be properly clarified by (or for) one or all of the suppliers of replacement plumbing--although it'll take patience and luck.
To me, one of the biggest issues for newer cars, including ours, is the transition from SAE to metric fittings and the implementation of the "global" ISO bubble flare joint design for brake systems, which is more difficult to fabricate in the field, requires a special flare tool that many may not have or are unfamiliar with, and in most cases is not something you will find replacement line stock at the local parts store in a "ready to use" condition (with metric end fittings or ISO flares).
The alternative is "do it yourself" or reaching out to someone like George Bates, for example, who created some replacement fabricated (with braided hose) transmission cooler lines using the production tubing--basically providing a new flexible element section of the original lines. What he was looking for was to have people supply him with cores--sending him back the defective cooler lines that came off their cars to be fitted with new hoses for the next customer.
The ironic part of this is that somewhere within the GM inner sanctum, there HAD to be prints/drawings/specs for a supplier to follow in order to produce the plumbing in quantity--I don't think this was something GM did inside their own plants. Finding that sort of information would be akin to discovering gold in your own backyard--maybe an inquiry to the GM Heritage Center would be in order, never know what it might reveal.
