I remember the dual exhaust option on the 305 CID V8 would make the engine sound like it has no exhaust has that been fixed yet
We have PBR! 99% of the work on it done by SergentFido, I only did some tweaks and fixed some issues. Not all of the bodies are converted yet, but this will be enough to set up realistic factory colors and start working on skins. Meanwhile, Custom Cruiser temporarily is a mix of old and new materials lol, this will be fixed of course.
Custom Cruiser has full PBR now, although it made some shading issues more visible and we are trying to fix them currently. Also I added PBR paint colors, they include all of the factory paints I'm aware of and most of the custom colors from old version of the mod (removed some repetetive ones).
If you mean in general, just read documentation, it's explained there. For the colors, you can see on the screenshot how it's done.
The authors of these addons would have to remaster them themselves. I have no time to help with that. But maybe some of them will still work with few tweaks since this is not a complete remaster and a lot is still unchanged.