Discovered that Ancor does make the marine battery posts in tin/copper.
So they are copper with an outside coating of tin to prevent corrosion.
Tin Copper Battery Terminal for 3/8in (10mm) Positive Stud | Ancor

Score

I would like to convert my GMs to a top post battery one day. No room in the cutty, unless I put the battery in the trunk.

i never had any issue with corrosion on the side post ones, probably a dozen gms i had but they went to top posts on the 07, sucks, i have to clean it a few times already. the only issue i ever had was with my 99 yukon. they have 2 big cables and this little washer between them, the whole arrangement seems to want to back out the screw after a while

If you have top post lead acid batteries, try using a thin felt pad washer or battery protector. The Permatex battery protector spray that Mike posted,looks like a good finishing product as well.
Pico No 868 | Washers | Pico of Canada Ltd.
The felt pads really help a lot. I am sure Autozone and similar places sell them.
I use them on my 4x4 with a lead acid battery, and no corrosion problems since.
With the Buick'* fully sealed AGM Odyssey battery, you never have to worry about corrosion problems.
I know AGM'* are $$ but when the 4x4 lead acid quits, I will be replacing it with another Odyssey.