I ran a 350 in my for years and it had a lot of power with only a few upgrades (intake manifold, cam, headers, Jet performance q-jet, HEI). Decent mileage on the highway too with my old factory gears and axles. So that wouldn't be bad and it'll be a LOT easier, esp. since you won't use it much.

If you can get that engine/tranny combo together everything will work properly and you'll have overdrive so you can gear it down a bit if you want, but it isn't exactly a bolt-in project going to the gen3. There's a lot more than the motor to consider.