Several pretty major modifications must be made to fit a Hydramatic into a truck that was formerly equipped with a standard transmission, if the job is done correctly. That includes changing the shape of the firewall area of the cab and fabricating a different floorboard cover, and adding a crossmember. The rear axle ratio is also different. Unless the conversion was done by a very accomplished restoration shop, the truck is unlikely to be worth anywhere near the asking price.
Jerry