swapping from 37 amp to 55 amp, direct swap?

On my 69 amx, the 55 amp is cheaper than the 37 but $15 at orileys. I get the jobber prices there so was wondering if I can upgrade to the 55 amp with no other modifications?

If you have a 37 you have the wrong alternator anyway. For your year they used either 35 or 55 - NOT 37.
The 37 came later, about 72 or so, about the time the 55 went away. So if it has a 37 then it's running the wrong regulator for that alternator unless someone changed it, too.

My point is - PLEASE check the tag and make sure it's a 35 amp with a red isolation diode. If not, it's wrong and you may introduce troubles. 

Good grief, the 55 is cheaper? Those people are NUTS. the 55 was far less common and is more expensive due to the larger rotor and stator. But then that's remanufacturers........ they toss all the parts into large bins, they do NOT sort, they have people sitting at tables pulling them apart, any output, any year, any model, and toss the parts all together in big bins. They are then cleaned, etc. and when the parts are ready, they pull parts out and assemble alternators..........and you may end up with a John Deere frame and sold as "will fit AMC" but the case/frame will not be correct in appearance for AMC.
That's how they do them so cheap. They use the parts that interchange technically or physically and build what they have the most call for. 

How do I know?
I've been there - I've toured those plants and watched how they operated. It's the same with carburetors - you'll get a reman 4300 for AMC and they've removed the transmission linkage lever and plugged the PCV hole in the back with epoxy and sell it for AMC. 

But your question is will it fit without problems - yes, rev is right of course. The larger alternator is deeper by about a half inch but that makes NO difference on the mounting at all.

But do please check your original as if it's truly 37 please double-check with us before installing the other. It will FIT but if someone has messed with it, it won't charge the correct voltage. 

You should have a 35 amp with the red isolation diode plate, single diode in that plate. 
It should be an A12NAM457 on it now and you want a  A12NAM605
The A12NAM604 is the same but clocked differently for the 6 cyl engines - so they may sell you something that's clocked differently making wiring a bit of a chore. 
You need something clocked at 12:00  meaning with the threaded adjusting boss hole at the top the brushes are also at the top. The 6s were usually clocked at 9.

Yes if it's for the same years, for example, 64 through 70, yes.
A 55 will indeed bolt in where a 35 was removed. 

It is odd, ironic, counter-intuitive indeed that the 55 would be cheaper or less expensive but then perhaps fr the remanufacturers and jobbers it's cheaper to stock one size...........

Now for the rest of the story - to be very very blunt - the only reason to do so is cost. 
There is absolutely NO advantage to moving from a 35 to a 55 unless you add AC, a really heavy-duty maxed out stereo or fuel injection.  None, zero, zip.
You will gain nothing in your case other than the cost savings. Nothing, not a bit, period. 
So if it's for some other gain, there won't be any at all. If it's to save money, go for it. We all love to do that.

If it's to save the 10 or 15 bucks then go with the cheaper since IF they have made the thing right at the remanufacturing facility and marked it correctly for the right cars, it will be a direct fit. 
Again, there's perhaps a half inch difference in the "depth" front to back but since the mounting is the same - in other words the 35 and 55 take the SAME PART NUMBER front frame half, that means it bolts on exactly the same. 
Depending on your luck you may get one clocked correctly, or may get one for a 6 that's clocked differently but the main difference there is ease of accessing the wire connections on the alternator. 
They are functionally the same, connect the same except for the position of the connections 6  vs 8 cyl, and mount the same. 

Will it make any difference in your lights, starting, or anything else? Nope.
Not unless the unit you have now isn't working correctly. 
So you'll save enough money for a decent steak supper, but it won't matter to your car at all. 
There is a myth that bigger is better, more is greater - not true. 

(I know 'em inside and out - it's what I do)

(By the way - anyone looking on wondering about how things compared, AMC to the others,  ALL ELSE BEING EQUAL - meaning comparing a 35 to a 35 or a 55 to a 55, the Motorola alternator puts out more at idle speeds than the GM or Ford of their day - until GM moved beyond the 10si that is.
Charts showing output at RPM show the Motorola puts out more at lower RPM than others.
This means that if you do a lot of driving in town, stop and go driving and so on, you are better off with a Motorola than an early Ford or GM alternator until GM upgraded the 10si and improved performance at lower speeds. )

The reason I was considering it is I've put bags on my wagon and the air pump pulls a good bit of current, and I just reactivated the A/C. When the air pump is on my voltmeter drops down pretty close to 12. The alt light has never come on, but it's more load than the 35 was ever expected to handle.

The pump is mounted in the rear of the car and I'm using a fused #10 wire directly off the battery straight to the pump. There's another 16 ga. that runs to the fuse panel for the electronics. I occasionally get a notification on the suspension control panel indicating low voltage so the pump shuts down. After a while the alternator seems to catch up.