What will fit in a 62 American?

At face value comparing an inline four of 2.5L or 153 Cu to a V8 of 304 cu in by saying they are rated at the same HP and calling it even is a bit like comparing apples to oranges.
The 153 cu in 4 cylinder engine is rated (depending on the year) at 119Hp at 5000 rpm, while the V8 of 304 cu in is rated at 125HP at 3200 rpm a significant difference in rpm will affect final gearing choices and transmissions.
For example, the V8 rated a 125HP at 3200 rpm will also develop according to the math involved, 205 Lb Ft of torque at that rpm.
The 4 cylinder engine rated at 119HP @5000 rpm will develop about half the Torque or 125Lb Ft at 5000 rpm.

Now while to some that is gobbledygook, it means that depending on whether you need grunt to move things or HP to get something down the road the curves are developed differently between the two engines and that is going to significantly alter drive ratio and transmission choices.
Frankly I build and think in terms of HP and have built and run 4 and 6 cylinder engines along with V8's. And about 1HP/Cu in is usually fairly easily obtainable using standard over the counter parts.
My 6's will easily pull 2.53:1 rear axle ratio's found OEM in the last of the AMC I-6 cars as standard issue and I use either a 5 speed or an automatic with a 2200 rpm stall and a lock up feature.
But to expect to drop in a 4 cylinder engine that needs 5000 rpm to develop it's comparable to V8 HP And have it pull a tall final drive is expecting a bit much.
It is not going to be easy to drop in a different engine in an early AMC car and one has to expect a compromise some where in doing so.
My preference other than staying with OEM and making it work, is to use a V8 and deal with changing out the front suspension to something like a FATMAN front end which gets things up to date and parts availability and putting sheet metal in the engine compartment to make room for a V8 after the spring humps are removed because the springs are no longer there.
And frankly for ease of updating the drive train? A SBC has all kinds of transmission options that an older AMC V8 does not have at all.
I have a 350 Cu In GM V8 with a 700 R4 installed in a Hornet running and drivable for under $1000.00 doing most of the work myself.
That was picked largely because of the options available for the package.
And it was selected over using a 232 with a 4bbl and a 5 speed or a 904 or a 360 with a 5 speed or a 904 both of which I also had. I do not regret installing it, the versatility has paid for itself at the present and the cost benefit is still way higher.
I dunno for each their own I guess, when I get the Sportabout painted and finished, the 360 will go in a hatch back. The rest? Will get sold. I guess.

Agreed on the 302  vs. 350 being an easier install. You can still use the stock suspension with new springs, about 14% stiffer than stock. I used 12% stiffer for better handling with the 196 and found that to be pretty much ideal. I'd go only one or two percent more with the V-8, as it's only 40 pounds or so heavier than the old 196. 

Wouldn't be hard to use the original suspension, quick and easy to just disassemble, clean, make a couple simple mods for better lubrication, assemble, and off you go. Lots more work to cut it up and weld in the MII, and would cost a bit more. 

Either way works, whichever way you go keep us posted! Will advise where I can...