Trucks and vans are the only vehicles that are worth putting an overdrive into. This cutting with the floor; okay, "crossmember" is not worth all the mess. If you want gas mileage then buy a second car; cheaper in the long run.
So buying, insuring and maintaining another car is cheaper than a tranny install? Can't say that I've heard that one before.
What if you actually drive your muscle car across the country instead of just around town but want to keep your 3.91 or 4.10 gears?