Yes I have had a problem with bolts not long enough before, not sure if it was on escort or sierra but just use a longer bolt ,But make sure it will tighten before the shoulder hits the treads.
use a meduim size socket and press to remove bushes if you want to re-use them as to small and they can tear and to big a socket will trap the bush against the sides and shear or tear it aswell.
The hole only needs to be cut in the floor if the single long bolt treads go and not the small ones as yes you can get access through the side holes.
Hole only needs to be big enough for a nut and socket. Then fit a rubber gromet.