If you want to switch to negative ground with your current generator, this is a very simple process. I've done it in the past, and I saved the following instructions since this question keeps coming once in a while.
Convert positive (+) ground to negitive (-) ground
https://morgan-spares.com/technical-help
Edit: corrected the link. Old one was not working
•Disconnect battery
•Switch wires on Amp Gauge (this is so the needle moves in the correct direction. No damage will occur if you don't change this)
•Switch cables on battery so that the negative terminal is now to ground
•Switch wires on coil so that the negative terminal goes to the distributor
•Re-polarize generator field: take wire from live (positive) source and touch it to field terminal 3 times. Should see small spark. Reconnect generator field wire
•If Generator is in working condition and Ampmeter fluctuates wildly, not showing + charge, then voltage regulator will need to be replaced. This happens very rarely but sometimes the regulator keeps its old polarity and will not switch.
If you're installing an alternator, the last two steps are not needed.
Please note that the starter motor will keep working as usual. It will not turn in reverse.
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Alternator Mounting Bracket
For installing an alternator, you'll need to modify the mounting base. The following kit will make this task very simple:
http://alternatorparts.com/alternator-brackets-1.html
UNIVERSAL ALTERNATOR BRACKET to install DELCO , FORD and CHRYSLER alternators in place of generators that have 2 mounting ears. Ideal for agricultural, industrial, and older vehicles. Bolts to existing 2 ear generator mounting bracket. Complete with hardware.
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5/8" x 2 1/4" (wide belt) pulley
http://www.just8ns.com/catalog/
Use the following number in the parts search: 98NAA5825
I'm currently using a 5/8" belt instead of the 3/4" that's standard even though my alternator has a 3/4" pulley (it's a Hitachi). It works perfectly. Getting a 3/4" pulley might be difficult.
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Alternator - Delco SI
Some theory: [url]http://bob_skelly.home.comcast.net/~bob_skelly/alternator_conversion/wiring_alternator1.html[/url]
These can probably be sourced at any parts store. At Rockauto, look with this part number: 3351094
It's a DELCO 12-SI, 78AMP (Corresponds to a 1985 Chevy pickup, V8
Note: Any alternator can be installed. The key with the Delco is that they have an internal regulator, so you don't need to worry about the extra cost or the wiring.