MY OPINION AND EXPERIANCE!!
Once you go five speed you will not be able to care less if the stock floor pan is intact EVER!!! The KEISLER 5 SPEED is the single best upgrade to my car. Even better than the 422ci small block.
If you sell and they say well the floor pan is not stock. Say lets go for a ride, take them out on the interstate if they still have concerns take them home and tell them to leave they are not being honest.
I did the swap over ten years ago. What I liked about the TKO was my 3.23 rear gears with the first gear in the TKO is similar to an 833 with 4.11. My cruise at 75 is 1900rpm with 275-60-15 BFG drag radials. I have been 170+mph with this combo AND at 70 to 75mph I have gotten better than 20mpg. Is it work yes, is it insurmountable no.