Survivor
(Savage Species variant, p. 89)

Those who survive a program of frequent assaults and other dangers emerge a few weeks later — tougher, faster, and less vulnerable to attacks.

Requirements
Base Save Bonus: Highest base save bonus must be lower than character level.

Special: The path of the survivor requires a concentrated month of study to enter the prestige class. During this month, the character must stay in one place and spend all his waking hours in study, practice, and meditation.

A booming voice says
“What qualifies you Rorek for having survived frequent assaults and dangers only to emerge tougher, faster, and less vulnerable to attacks?”, Rorek thinks for a moment, and by that i mean takes a big long drink from his Jamas mug and responds with … “I’ve been thru Dregnoths Labratory more than once, and lived, does that count?”

Note, that it would be silly for each f you not to take 5 levels of this class as part of your 5e to 3.5e respec since I said you can start another class at 5th.