I made my opensprinkler also on a Mega 2560. I have a post here: http://unpuntilloalambre.blogspot.com.es/2013/01/opensprinkler.html (sorry it’s in spanish). It is not actualized, but I added a 8 relay board from ebay.
Usually I take the code from firmware 1.x and it works perfectly, but I am not able to upgrade to 1.8.3, even when I flash the new code the displayed firmware is always is 1.8.2 . Any idea?