Will it interchange

Tell him if he wants gas mileage, swap the rear gears to 2:76s instead.
By installing the O.D. tranny and all of the fooling around associated with it, it will make his car a turd to drive.
The ratios are all wrong, as in first is too low and the rest is a pretty wide spread between gears. He might be able to pull it off because he has some torque.
If he plans on doing burn outs a lot, the O.D. tranny won't last especially if it is an aluminum case one.
In the long run, he's looking at gaining maybe 2-3 mpg on the highway and around town nothing. That is unless he drives it like a grandma, and what is the fun in that when you are driving a Road Runner with a 383?
Tell him to buy a new Focus if he wants gas mileage and be done with it.
JMHO, and my opinion only.