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1939 Packard Six. Contemplating an engine swap. Suspect the stock rear axle would not respond well to torque and horsepower numbers in the 300+ range. Can the axle gears be replaced with others with a higher power rating? Or is the best option to just replace the axle with something like a Ford 9 inch? Any insight would be appreciated.

I would think you would want a different rear end with better gearing. They were geared for the power output of the original. motor and the speed of the original roads.

This is why people had or added overdrive.

You make want to use a later rear end with either a 3.23 or 3.54 gears.

Plus you'll want bigger brakes to pull the reigns on all those horses,,,

Big Kev,
Thanks for your suggestions and a reminder of just how much travel by car has changed. These older vehicles are definitely time capsules.

Flackmaster,
Thank you for pointing-out the limitations of older brake technology, I appreciate your comment.
This Six, a Business Coupe, was restored by a previous owner, and I am going to evaluate the effectiveness of those period hydraulic brakes. My desire is for this car to just be a cruiser - no racing about. Just getting on a modern two lane highway with speed limits of 65mph is going to be hairy enough.

All well and good -- but then I don't understand your comment about wanting a 300 HP engine. Highway driving is problematic for old cars more because of engine speed than engine power.

The 300 HP 1956 cars could hit 120 MPH, and since required power is times the cube of velocity, this implies you need only about 40 HP to cruise at 60 MPH. Of course you need more for hills. But still, more a function of gearing and having increased motor life from not flooring it, than a strict lack of power.

Point is you could swap in something marginally better, e.g. a 150 HP straight eight from the 50's, and the car should be just fine if you have an overdrive and/or a decent rear end ratio. Without needing to beef up or re-engineer the entire drivetrain.

I appreciate your well-reasoned comments. My target of 300HP is based on past experience of what that amount of power can do in terms of general performance. I did not mean that in a written-in-stone sense, just a base for conversation. My intention is to enjoy an occasional drive into town (less than 10 miles), likely fewer than 500 miles per year. Returning to my previous post, safely getting onto a busy two lane highway, specifically US 97, requires the power to quickly blend into traffic usually travelling well above the posted limit of 65mph. Out there, there is no margin for error. US 97 is the second busiest N/S highway in Oregon, and the most popular truck route, rivalling I-5. It is also something of a slaughterhouse; I should buy stock in companies that make flat-deck tow trucks. Using the relative performance of my 5.4L Triton-equipped F-150 as a unit of measure, 300 horses seems about right. The fact that this Packard's smooth-flowing body lines have street rod written all over it does not hurt. A shiny new V-8 crate engine seems the easiest way to meet performance requirements for modern roads and to stay safe. A straight 8 would definitely work, but I have no illusion about availability and pedigree of any such engine I may find.

My shop and most of my Packards are also "in the boonies" of Oregon off Hwy 97. Although I've not driven them there, when they were here in California as everyday cars, they had no trouble at all in highway traffic. In fact, the main problem was in town where other drivers would see an old car and pull right out in front of me no matter how fast I was going! Fortunately Packard brakes are good too.