swapping from 37 amp to 55 amp, direct swap?

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 Driving at night with lights and heater on is more load than the alternator can keep up with at idle on many cars. The headlights will visibly dim at idle as system voltage drops and current is drawn from the battery. 

So I'm going to ask - what college teaches that, these days?
And how many cars have pulled into your shop where you measured system output, loads, etc. and determined this to be the case??

I disagree a lot. Decades of experience with my own vehicles and hundreds of customer cars, plus college shows that if you see your headlights dim while running the heater, you need to have things checked out. If you know of many cars like this, send them my way, please!  
I can go out right now and start my Javelin with ORIGINAL wiring, original type headlights, and the original, untouched alternator - turn on the headlights, fire up the heater blower (and my car has AC so has a strong blower motor) and not see the gauge in the dash drop below either a slight charge or sitting at the null spot - center meaning no discharge. 
If your alternator isn't keeping up and you get say, a drop in voltage of about 1.3 you lose nearly 1/3 of your light output - THAT is noticeable. 

the folks posting in this thread aren't talking about a very slight discharge at idle, or voltage dropping back to battery rated voltage of 12.6 or so, they are talking more........... and in that case continuing that sort of operation is bad. 
The fellow with the bagged car and compressor and AC needs a larger alternator. 
Now the other with a 35 was simply asking if a 55 would be a direct fit - he's not got a problem, just wondering. If it's cheaper or even the same price, and there's ZERO work, no modifications needed, I'd guess - why not?

In any case to the thought that it's normal for things to not keep up on many cars -  no - lights and heater should not be too much - the factory accounted for that. If it's too much then you need to be checking things that have gotten old or worn. A slipping belt, glazed belt, worn brushes, weak diodes, wiring issues - poor connections, bad grounds, etc. 
Yes, you may notice a drop in headlight output as the system settles from the 14.5 it was putting out to the 13.5 it puts out at idle if you are loading things, that's because headlight output drops severely with voltage drop, but it should not be pulling from the battery at idle. If you are basing a comment on seeing lights dim -then yeah, even a slight dip will dim headlights with the inadequate 18 gauge factory wiring with single point of ground - but that's not alternator not keeping up.....

Here is why I mention that and suggest one does NOT use headlights as an indication but should use the volt meter or amp meter instead (headlights are not a great indication because they drop output severely as voltage drops)
Headlight output drops severely with decreased voltage. The drop in output is exponential, not linear.

10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens  <- A bit over 1 and a quarter volt drop and you reduce output by nearly 1/3 ?!
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens <- output rated at 12.8 volts
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens
14.5V : 1528 lumens

If you don't match the alternator with the load, or the load with the alternator, you will be taxing things beyond norms. 
On the other hand, if you have not added a constant or running load beyond factory and want a Summit 100 gigawatt wireless alternator simply because it's more - that's just as bad because it's not logical and is an emotional decision, based on being cool. 