I use ACF50 products on dissimilar metals. They use it on aircraft carriers so it must be good. I've found it on Ebay in the past.
http://www.acf-50.co.uk/corrosion_block.htm
Also comes in spray cans. Don't spray anywhere near brake discs though. It does not help braking (as a mate found on his GSF600)!
I have a spare rear beam from a low miles 05 car if you want one? Going for scrap soon if not?