Here's another thread on this topic:

http://www.peachparts.com/shopforum/tech-help/81253-300e-transmission-question.html?highlight=722.358

Here is a company that says they do the valve body modification - you remove the valve body and send it to them (call first so they can schedule it)and they will rebuild and modfy it and ship it out in 1-2 days. They charge $275: Silver Star Transmissions (or, SST), 214 West Edmond Rd., Edmond, OK. 73003. 1-800-369-6601.
I have not used them, but am planning to later this spring.