232 Swap to 360 or 401 Question

Bigger isn't always better. What is your intended use of the car? That should be your first consideration. An EFI 4.0L has the horsepower of a stock 74 360/4V. Not quite as much torque, but not a whole lot less. You get quick and easy starts and parts availability, and it bolts right in where the 232 was (use the 232 mounts and plates off the engine). You can use the TF904 trans, but an AW4 OD auto would be better. Of course you'd need a rear axle gear change to fully use the OD. Six cyl Javelins with auto or three speed man came with 3.08, you really need 3.31-3.55 gears for the AW4... 3.31 was an optional gear, maybe you got lucky and have those...  With a carb it wouldn't put out quite as much power, but with proper tuning would be close. You'd lose the "start and go" ease of the EFI, but with a small 4V, Offy DP intake, and a mild bump in the cam you'd have near stock 360 power even with a carb. A 258 would make the same with a few more mods (bump compression up).

If you plan on using it as an enjoyable cruiser with a little spunk the 4.0L EFI/AW4 (or better, a T5 five speed... not the Jeep five speed, but that would be okay) combo would be a great choice. I do stress stock 360 power. You can't build the 4.0L to put out a lot more power than stock, not a lot of "low hanging fruit" -- not many easy power adding things to do. You can squeeze maybe 30 more hp out of it. A stock 360 has the potential to make a good deal more power without too much expense. So if a wheel burner or hot street/strip car is your focus you need a 360. On the 4.0L you have to start adding things like a turbo or supercharger to get a lot more power.

The AMC 15 will handle around 350 hp easily. It's not as weak as people seem to think. Like all AMC axles the hub nuts need to be loosened and re-torqued to 250 ft/lbs every 10 years or so. If you bang on it hard, such as drag racing, it won't hold up as long as an AMC 20, but will still go a good bit before breaking something. The four wheelers break the similar Dana 35 on occasion, but usually when a spinning wheel suddenly grabs. I ran an AMC 15 behind my 4.6L stroker for a while, never a problem. Others have run the AMC 15 with hopped up 4.0L in cruisers that they "get on" on occasion with no issues as well. I'd run one in a hot street car, but probably wouldn't abuse it too much with slicks and a good hook-up. Street tires (unless really wide) will break loose before putting too much stress on the axle.

Javelins with six should have come with the AMC 15, just like the Americans and other small cars. Only the big cars got the AMC 20 with the six (232 and 258 only... 64-65 with 195.6 got the AMC 15).