Here's the deal...No matter what the weight is, the Slant 6 is not going to fit in that long narrow engine compartment.
It's also not the best engine for the car. Here's why.....

As far back as the '20s, the Nash was a big car. It was powered by a decent sized overhead valve six. I believe the architecture of this six was carried over into the '50s. I will try to attach a picture of the engine, maybe two. The '50 Nash engine was 234 cubic inches.

As the car manufacturers thinned out, Nash became part of American Motors and I wouldn't be surprised to find that the Rambler six that we knew in the 1970s as the AMC 232 and the 258 were somehow related to the original Nash 6. They were available in AMC cars with a Torqueflite.

When Jeep was purchased by AMC the weird assembly of engines Jeep had been using were phased out and AMC engines were used to replace them. The last gasp for the AMC 258 six was the 4 liter 6 and 2.5 liter 4. The cylinder head from the 258 was redesigned and is much higher performance than the 258. The crowning touch came when the 4.0 was given fuel injection. There are hundreds of thousands of these engines running around and many if not most of them are bolted to Torqueflites.

Along the way, the crank from the 258 was used in the 4.0 for a stroker by the aftermarket and us hotrodders.

To my way of thinking the fuel injected 4.0 or its stroker variant is what belongs in the car, because of its heritage. Also weighing in on the 4.0s side is the configuration, the rock-solid durability of the engine, the wide availability of parts if any are ever needed, and the fact that it looks a lot like the original engine.

If you want carburetion, the 258 is out there in the same physical package.

R.