I doubt you'd have much corrosion problem on a trailer. Dissimilar metal is more of an issue on stuff that lives in and under saltwater.
Longer brackets are the ticket; you may have to change the design of whatever is on there now, though.
My trailer has risers that are a square U shape with a slot lengthwise down the middle, maybe 12, 15" long. A U bolt goes around the trailer crossmember, and a plate on the other side facilitates bolting it on. The slot allows for adjustment. All hot dip galvanized. A single bolt hole at the top allows for the tilt of the bunks to be adjusted.
On the bunks, a fairly long support bracket bolts to the wood (extends about a foot to either side of the riser attachment), and the connection to the riser is at the center (I need to have some new ones of these installed, as these are rusting out at this point.) In addition, to get a bit more height, I bolted in an additional piece of 2x8 between the bunk and the brackets.
Here are the risers I have (or similar, anyway):
12" Swivel Top Bunk Bracket Complete 1/2" - Champion Trailers
I haven't been able to find the support pieces online, may have to get them made up by a metalworking friend.