If you're having to put in rockers and floor pans anyway, probably torque boxs too, you may as well use the vert pieces. We did this on the 68 Fastback, without changing floor pans and it was not really all that difficult. You can also use the vert underfloor pieces, but you loose a lot a room in the tunnel for things like exhaust and driveshaft clearance. Not a big deal, you just have to be a little creative with the exhaust.
David.