I have a 99' FXDX. For suspension and chassis mods so far I have Progressive fork springs with 15wt oil. Sputhe chassis stabilizers. Dunlop Elite tires. Bike handles like it is on rails pushing it thru the twisties and tracks true on the highway.

From what I hear a fork brace is a must on the 39mm forks. What can I expect with one? Is it worth the $?

 

Fork braces were big during the 60s and 70s extended fork tube era where there was a lot of flexing. On a conventional stock front end it will make the smaller tubes slightly more stable. Noticeable probably not. Worth the money depends on the price, your finances and your riding skill to notice a possible improvement.
Looks cool and at the right price it might be worth it for that alone.

 

"on rails" handling prolly dont need one