For the money spent it's not worth adding a final. There are so many new radio's out that have so much more power anyway, sell it and get a different radio. The 3900 comes in a dual final version.
As far as the 2290 mod, you will just have to e-mail or PM the techs on the forum and see if they will help you. Again though, it's such an odd modification that it would cost a bunch. Then you might be three states away and have a problem, so you take it to a shop on the way and they have no clue how to fix it because they don't jnow what and how it was done.
Heck this lower powered radio would work well with that 3 pill base amp your looking for so why mess woth it?