Valve covers
 

https://cimg8.ibsrv.net/gimg/www.cor...ae4571c35.jpeg

Does anyone know where these come from?

I found them on eBay.

Any suggestions on cleaning them up better than start with 400 grit and work my way to 3000 then polish?

Cal Custom probably. Aftermarket dress up item.

Bead blaster....low pressure. (Sand blaster is too aggressive for aluminum)

Before you start sanding, pick up a can of this and give it a try. Works wonders on older trim pcs etc. tear out a pc of the wadding and start polishing (wear some gloves) wipe clean once the dirt comes up. Avail at most auto parts stores. If it does not work, I might try sanding but would start with wet sand with 1000 or finer... or even a Brillo pad.
https://cimg2.ibsrv.net/gimg/www.cor...e4e2a7290b.jpg

Starting sanding tonight, they are a bit scratched up so I think sanding will give the best finish. It is just going to take forever. Luckily engine dress up is probably going to be this winter. I am just gathering parts now.

400, 800, 1000, and mother’s gets the section I did just shy of mirror. I may add 1500 next time I sand and see how it goes.

Check on the inside to see if there is an 8 digit number cast/stamped into it. I could be wrong but those look similar to covers that I saw in the Chevy Power parts manual back in the 80's

Another good product to use is ( Blue Magic) Polishing cream, works very well on chrome,Aluminum and mag wheels.
as said above wear gloves.

One idea
 

https://cimg4.ibsrv.net/gimg/www.cor...1c8c9eaf5.jpeg
I am not able to get a uniform finish around all the books and crannies. So how about some paint!

Dude - Looks very nice