I used some of that double sided foil covered insulation which you can but in the big box stores. Does it make a difference? Can't really say it does. I know the hot rod suppliers offer a similar material but much more expensive. I have old pieces of fabric carpet on each side of the hump and I have some old rubber floor mats on top all covered by what I believe is the original rubber mat. In my 53 I had a repop mat which fit very well but was almost paper thin. I would not go that route again. You can make a pattern of your floor from cardboard and get a piece of rubber and make your own.
Incidentally I also put that double sided foil material under the door panels.
