I've done Forney down to 70 and back up Forney to clingmans spending 1 night at 70 and one at 69 towards the end of may and it was good.
Duckypaddler and I did a one night trip down to 69 and back up last October and it was a good trip as well... A little colder but we still caught fish so I would say upper Forney would probably be in play but really dependant on conditions, like a late cold weather spell. I will say the better fishing on both trip was above 70.
If low elevation is key, you could do a short hike to 93 on twenty mile and hit both twenty mile creek and Moore springs branch. Smaller rainbow water but not pressured too much and low enough elevation the temps may be good.
Another good base camp would be 39 in cataloochie or a loop on the rough fork trails. Higher elevation so watch temps.