Welcome aboard. You don't want to build that 235, the engine you want is a 216. (that's a joke, and a long story.............don't ask..... ) Actually, the 235 of that era is a pretty good little engine, and will respond to the slight mods you are planning. Yeah, I think a 390 is a better choice. General rule is 100 cfms per 60 cubes, so 235 works out to just about 390 on the nose.( of course, I'm running 2 x 500 cfms on 306 cubic inches, but I'm not good at math )

I can't answer your questions about the Clifford stuff, but others on here will. However, I can address the ceramic coating. If you are talking about HPC and some of the others, they are great IMO. My Limefire headers were coated 15 years ago, ran the car on the street daily in rain and everything, and they still are presentable. Plus they coat the inside too, so you don't get rustouts.

Until you get the engine dialed in, don't install your shiny coated headers. You run the risk of cooking the coating if you are running too rich or lean. I slightly blackened mine because I was too impatient and wanted to hear my rod fire up for the first time.

Don