The only problem with the Aussie Chrysler six is that it's in Australia -- hard to get over here! Domestic choices are the AMC/Jeep 4.0L, Ford 300 (4.9L), and the Chevy 250/292. Personally I'd use the Jeep or Ford six and keep the EFI. The Ford will be easier to find in a light truck with rear drive trans. Jeeps are hard to find in 2WD if you're in or near snow country. The only GM option I'd consider is the recent OHC I-6 or even the I-5. Any of the three would make a good driver getting around 20 mpg with an OD transmission (auto or manual). The Packard will have a LOW final drive gear, so an OD trans is a must. Hmmm... will probably want to change the rear axle out too, the Packard probably uses a torque-tube setup.
I'd definitely make the mods bolt-in and keep the original drivetrain stored. The car would definitely be worth more restored, but driving a car with such an old engine daily isn't for the feint of heart! I've done it (the old Rambler 196... some parts hard to get!) and been stranded for a few days a time or two waiting on parts, luckily only once on a long trip. Head cracked then... was costly to get the car back home!
A universal small block engine crossmember can be modified to fit one of the sixes and bolted in. Just a matter of a few holes drilled in the frame, and they can be easily filled if a later restoration required it.