Best guess is the "application". As referenced in the below post, Holley changed the label for the D100 headers to '72-74, while they will fit up to '89 or something and last I checked they are still available. Not sure why they couldn't do the same with the A-Body headers and change them to '73-74 or something. And it sure seemed like I saw a fair number of LS swap headers on the clearance page for awhile, so I wouldn't be surprised if more than we think went away.
Not really sure what direction I would go if I had to start over or didn't have my swap parts already. I really like the fact that the Holley mounts allow room for things without cutting or fabricating, but have to weight that against being able to buy headers. I can see a ton of work required to fit a low mount AC compressor, and I much prefer that setup to the Holley FEAD kit or a high mount one. The SRV sure gives a bump to the top end but is it worth cutting the firewall on a real 360 4M car? The oil filter is a whatever thing, I don't see the solutions as extreme. Not sure, but I could see running manifolds and giving up whatever power they cost. The other options are a bunch or work or a $5K HDK kit.
