The "problem" with parts packs is that the production, packaging (design and production) shipping, warehousing, cost are about the same as for a normal production kit, which you can buy for a few bucks more.
IMHO extra features in standard kits is the way to go, mix and match something which can be promoted through, an instruction sheet, the side-panel of a kit and even the website or facebook page of a manufacturer, which today are kinda boring, and should draw the visitor into the fantastic creative world of model kit building, to build your dream.
About the different versions of the 32 vs. the '29-30 Ford's, messages I see here..kits today are designed in 3D, of which the files are sent to a tooling company which translates that data into hardware, which then goes to a injection molding facility to produce shots. The point I'm trying to make is, the 3D files are valuable because there's the starting point for alternative versions, tooling can be designed to incorporate alternatives, but it's more common today to "cut" new smaller tools which form a cluster with the "main" tool to eject alternative versions.
Just my 2cents