Were you able to get a good resolution for Authorize (dot) net [sorry SPAM filter kept blocking the post] I found a post in their development community about source code on Github that apparently someone has working with 2.5 using some basic PHP code changes and an updated SDK. If you are also still searching this may be worth a look. My client doesn't have the resources to test it out.
From there you can find the Github code. I can't do another link or SPAM filter blocks me until I get verified as contributor.
If anyone has a good solution, please reply!