A904 Hard 1-2 Shift and Reverse

I recently acquired an "All Original" 1969 Barracuda Convertible 318 2bbl with only 68,000 miles on it. Rebuilding the entire front end (It's toast, everything rotted and loose) with a front disc brake conversion, manual brakes, steering and 3 speed A904. I am experiencing (cold or warm) hard shifting into reverse and from 1-2 especially accelerating slowly. However, going faster helps but is still hard shifting just not as much. I have never rebuilt a trans but I know it is in my near future. Could an off the shelf fluid additive prolong the trans life for now AND what is potentially causing the problem and what am I looking at replacing? Y'all are the experts, I am a newbie who is relying on your knowledge and experience to guide me to greatness. Thanks