Probably, but I'm not sure if there were any little differences between the '97 and the '99 that would cause issues regarding other equipment on the truck. If I were you, I'd download the '97 and '99 service manuals here and compare pinouts for the various connectors. This would help prevent you from running into anything unexpected and give you an opportunity to compare things in general. Which leads me to the reason for my original post...
If the '97 harness for the 305 setup is basically the same as a '97 with a 350 (I'm betting it probably is minus the different O2 sensor configuration and maybe some other little bits) it might be easier to leave it in place and just add what you need, then tune the '97 PCM for the 350.
Speaking of O2 sensors, do you have the exhaust off the '99? The 305s shared an exhaust with the V6 trucks and only had one cat IIRC, so they have three O2 sensors vs four on the 350 setup.