I personally don't like the look of a round tank on an older truck. Unless it is a really big truck and came with a round one originally.
I have a picture of a 52 or so Chevy 2 ton with two smallish round aluminum tanks from a peterbilt. I don't like the look, but can send you the picture if you would like.
I personally am going to use a rectangular aluminum tank from a Freightliner for my 48 2 ton. It won't be quite original but I think it is a nice looking tank and should go well with my aluminum bed. Check the pictures on my link, the last few in the Detroit folder.
Grigg