One problem you might have is that the engine bay was designed for a straight engine. So the engineers may have put things in that will interfere with exhaust and such on a V-8. But since you seem to have an economical cruiser in mind, I'd look at a modern straight six. There are really only two -- the Chevy Vortec OHC, which is so new it might be a bit expensive, and a Jeep 4.0L. Cherokees are pretty common in salvage yards now and would be a good compromise between power an economy. They are rated at 190 hp and should have enough torque. Finding a 2WD Jeep in the north is pretty tough, but they are common enough down south. Using the OD auto from a 2WD Jeep would be preferred and the easiest to install. The wiring isn't difficult to use the EFI. The stock axle can be used if it's an open driveshaft, but forget trying to convert a torque tube.
Gearing is based on the trans and engine torque at low rpms. Most of those old early 50s engines are ready to pull off idle, and have a low, flat torque curve. Many have low geared rear ends. The eary Ultramatic used the torque multiplication of the torque converter to full advantage. It slipped a lot taking off, but that slipping allowed it to multiply engine torque quickly, with the side affect of creating a lot of heat. Most US torque converters produce a 2:1-3:1 torque multiplication at low speeds, meaning that if an engine puts out say 100 lb/ft at idle, once through the converter it equals 200-300 lb/ft. Then you have the gearing to add to that.
The Packard is about the same weight as a Jeep Cherokee, so you should be able to expect about the same economy with the same drivetrain (4.0L, OD auto, 3.55 gears = low 20s on the highway). I get 22-24 with my stroked 4.0L Jeep six (to 4.6L) in my 63 Rambler wagon (see my album). When I ran a stock 4.0L I got about 2 mpg more (24-26 average cruising on the interstate).