The first issue you need to address is the alternator output of your old Ford. I've owned several 60's Fords and they typically have very low output, light duty alternators, somewhere around 35 and 65 amps. Heck, you might even have a generator, which is worse. My '63 ford has a generator and the output is variable with engine rpm. This is bad news for sound systems, so if this is what you have you should upgrade to an alternator system.
In either case, your alternator or generator is most likely going to be completely insufficient to run a sound system with a power amplifier and sub woofers, particularly at night with the headlights and other electrical equipment on. You will need to invest in a high-output alternator. At this point I cannot say what the output should be because we don't know what equipment to install, but suffice it to say that you cannot have too much alternator power. The electrical system will only use what it needs. Also with these old cars, you might need to invest in ignition shielding and other noise suppression gear. Old Fords are notorious for generating a lot of RF interference. When I fire up my '63 Falcon van in the driveway I can hear the static on the radio and TV in my house. That's how bad it is. Needless to say, the old van doesn't have a sound system in it yet and won't until it that issue is taken care of.
So now lets talk about equipment in general. There is a very large selection of decent audio equipment out there, and there is an even larger selection of junk. Although this isn't a foolproof method, if you stick to equipment that is "CEA 2006 Compliant" you are more likely to at least get truth in claims about power and performance ratings. The CEA is an international organization that has been trying to implement testing standards for electronic audio equipment to help reign in the extravagant claims that the less scrupulous manufacturers make about their equipment. False, highly exaggerated claims based on non-standard test procedures are widespread throughout the industry. When you see high performance claims for a cheap price - the 1000 watt amp for only $69 and 500 watt speakers for $29, for example, run away. Decent electronic equipment that is going to hold up to hard use and last awhile is never cheap.
For 4 full range speakers, a decent sub, a 5 channel amp, a CD/Radio head unit, mounting hardware and wiring, you're looking at spending in the neighborhood of $1000 to so for middle of the road price range, good quality gear. If you only amplify the subs with a 1 channel amp and you shop carefully for deals, you might be able to pare that down to about $600 to $700. CEA 2006 manufacturers include JL Audio, Rockford Fosgate, Kicker, NVX, Kenwood, Massive Audio, Alpine, Pioneer, Polk, Hifonics, JBL, Clarion, JVC, RE Audio, Sony and Audiopipe.
A couple of points to remember. When evaluating power ratings, refer to average power which is given in watts RMS. Ignore peak power ratings. They are irrelevant except for marketing people who use them to make the equipment ads sound more impressive. Another thing is that twice the power does not mean twice as loud. The relationship between power and loudness is logarithmic. In order to double sound output, you need ten times the power. For example. An amp and subs rated at 1000 watts rms is approximately only 20% louder than an amp and subs rated 500 watts rms. To be twice as loud, you'd need equipment rated 5000 watts.
What this means is that you don't need to be obsessed with power ratings. Lower powered equipment of good quality can be plenty loud and sound excellent, and when it comes to subwoofers, the cabinet is the main thing that will determine how it sounds. A good quality, custom-made cabinet designed for a specific sub will sound far better than a cheaply made, generic cabinet that is not sized and tuned exactly to the subwoofer specs. Lastly, for an old car such as your Ford (Galaxy?), you'll need to invest in some good quality soundproofing. The quieter you can make the interior, and the fewer potential squeaks and rattles, the better your system will sound. Do not cut corners in this area. Good equipment requires good sound proofing.
Well, this has run on far too long already. Two websites where you might want to research equipment are http://www.sonicelectronix.com/ and http://www.crutchfield.com/ They also have pages with all sorts of advice about selection and installation.
Good luck.