Brandon,
Good call on the 304. you will like it because it has good chug. twice the cubes of a L134 Willys 4-banger.
Here is why i like the 304 engine:
I have been running a 304 2bbl carb in my '77 Renegade since i purchased it in December of 1981 with 60k expressway miles.
The Renegade is a CJ-7 and it got a T-18 granny trans implant by me assisted with a Novak kit in '83, then carried me in and back out of the Rubicon trail for 16 summers. I got lucky and never broke. 2001 was my last Rubicon trek.
Anyway - in my now 39-years of ownership i have never removed a valve cover off of that engine. In my opinion, i would give AMC high regards for their engine. I fought overfueling while crawling on the Rubicon until I learned to drop the float level 1/8 to 3/16-inch (eye-ball) when hitting the trail head. The switch is quick because the top of the motorcraft 2bbl carb comes right off for quick adjustment of the float tab. I have never changed carbs in all the years of ownership.
Now, if _ _ _ you don't raise the float level back up for driving home, you will encounter a slight stumble pulling away from a stop light.
A stock amc 401 looks like a 304 and would really perk-up the comanche with great torque.
Paul,
The swap you mention may be the easiest and probably most affordable because I see inline 4.0L six's often on Craigslist for very cheap and affordable $$$$. and, you may wind up with EFI. win win