1973 C10 Pickup according to RockAuto.com

Part Number: REMY 20040 for the 42 amp one, which is the best one for the job really because it's below the 50 amp ceiling.
http://www.rockauto.com/en/moreinfo.php?pk=38314&cc=1029967&jsn=805

The NAPA part number is 13-4010.

Cletis's link shows only 63 amp or higher which isn't recommended unless you are bypassing your stock ammeter.

To hook it up, connect the Main Terminal to the Ammeter Main line to the (+) Battery. The two terminals that are to the side are 1R (closest to the main terminal) Charging Light, and 2F the one beside 1R is for jumping over to the main screw terminal. This is necessary for excitation. Just a 14 gauge jumper wire is all that's needed. Look at that link Cletis gave you though.. lots of good information there. The link to my website above shows how to wire it as well.

Deve

1950 Chevy 3100 Deluxe Cab
1950 Chevy 3100 Standard Cab
In the Stovebolt Gallery
The Think Tank
More info and tips at Deve's Technet