Question on Gearvendors overdrive . Is it a basic bolt in operation for a

1970 Dart , 340 with a 727 ? I drive a lot of highway just to get to the track 45 miles at 3200rpm is just killing me . With the O/D i figure 2000 or less highway and great at the track with 6 speeds .
Are there mods to the floor pan on the A body ? And would you buy one 2nd hand .
Thanks , first week with the car , thought it would never get out of storage with the weather we have had .