swapping from 37 amp to 55 amp, direct swap?

they are showing a direct replacement at Advance Auto that's 55 amps. Doesn't really give a brand or model. Is this the 10si you're talking about?

dogbone (you seem to be better than a dog bone, but whatever) no- the light won't glow necessarily with not keeping up - that's more in cases of where there is an imbalance in the system.
In your case it appears the whole system is LOW - meaning you pull the battery down, the alternator can't keep up with the combined needs of the whole system so it's all low "together".
(of course it's possible that one reason it can't keep up is the same thing that could keep the light from functioning, so that's possible, too!)

If you have a decent volt meter - not a meter in the car, but even a WalMart VOM meter for 20 bucks, you can run some simple tests if you wish...... I'd hate to see you swap parts and find there's some other thing going on or we could PROVE it's the alternator not keeping up.....

Let us know and I can walk you through some tests that you can easily do with a decent portable volt meter and it won't take but a few minutes. 
If I recall - and my memory sucks sometimes - your car has the alternator in an easier spot to get at than a later model with the second generation engine - assuming you have the 64 engine in your 64.

In any case, a 55 amp Motorola up through 1970 will bolt right on, and if you really wanted to keep it original looking and go BIGGER, I have wondered, thought about it, and bet if I tried I could build a 62 amp Motorola with an isolation diode.  ;-)

billd wrote: 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.

Its a 35.... sorry for confusion. Has a blue motorola tag on it.

jl2556 wrote: billd wrote: 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.  Its a 35.... sorry for confusion. Has a blue motorola tag on it.

No problem on this end, I just wanted to be sure that someone had not caused you problems by putting a 37 on it and switching the regulator (and the connector) meaning if you did put a proper 55 on it, it wouldn't work right. 
In this case it would work fine, no problem. Again, just wanted to be sure you really meant 35 and not 37, etc. as one is for earlier years, one for later, and they used different regulators. 
But 35 and 55 used the same basic regulator prior to 1971.

dogbone wrote: I put the typical set of gauges in when I rebuilt the motor and it's showing 11. I've noticed it has read low periodically, but usually is normally running about 13 or so. I know my battery is a weak point. It's probably 5 years old. And yes I scavenged it from another vehicle. Probably need to start there and have it checked.  I have a Klein digital multi-meter if that would work? Original look is not a huge concern, I'm thinking the Sanden compressor, aluminum radiator, among other things has probably already killed that. Troubleshooting would be great!

the Sanden AC doesn't bother me a bit - it's a decent looking piece and doesn't really interfere with teh appearance of anything else under the hood........... but that's just me. 

OK, a couple of things we can check fairly easily are the output at two different points at the back of the alternator.
Engine running, fast idle (say about 1200 RPM with no accessories, no AC, no air pump, etc.)
Ground the neg lead of your meter.
Take a voltage reading at the output post on the back of the alternator. 
Now take a reading at the "AUX" or regulator post on the alternator - it's where the ORANGE wire connects to the alternator. 
You should have 1 volt difference - if at a fast idle, NO load at all, the output from the post on the isolation diode is 14 volts, then the reading at the post or terminal the orange wire connects to should be 15 volts. (.8 to 1.2 is a range but I find if it's much off .9 to 1.0 difference you may have an ALT light glow)
If there is no difference you have a shorted isolation diode, if you have a higher difference, you have an isolation diode going open. 

Next, engine off, key off, unplug the voltage regulator at the regulator connection (not at the alternator)
Connect the volt meter to the alternator output terminal at the alternator.
Start the engine.
Using a short jumper wire at the harness regulator connector, connect the green wire (which goes to the field connection - the brushes - at the alternator) to the yellow wire in the harness regulator connector. 
The engine should "pull down" and the alternator output should jump to 16 or so, depending on the battery state, etc.
Don't run it this way very long at all.
It will either full-field the alternator and force it to max output, or it won't...............
If alternator output doesn't spike, there's a problem. 
If your car has the older system and a 3 wire regulator connector you may need to jump orange to green..........
The idea is to supply the alternator all the field current it would accept and see what it's able to do 

That car can easily take a 55 and not have to change a single thing.
You could even mount a 62 amp Motorola there if you wanted to keep things simple, with a 62 you'd either need to switch to a later regulator (and the connector to go with it) or have one built that does not have the diode trio inside but uses an isolation diode instead. 
The 62 could be built with spacers in places of the trio inside, and an isolation diode fitted to the back. 
But going from a 35 to a 55 is already twenty more amps......... so a 62 is likely overkill.