OK, so I know I did this the hard way and should have gone with the GM one wire alternator but we all have our reasons for taking the long road. In my case, I happen to have an alternator from a '83 honda accord that was given to me with the truck and was told that it would fit. My own poor judgment led me to take the guy for his word and I didn't really look into it before I got started on the project. The GM alternator is HUGE and I don't currently have the tools and supplies necessary to fab the custom brackets and figure out what to do with the lower radiator hose. Anyway, here's what I did.
The alternator from an '83 accord has very similar mounting tabs to the stock 620 alternator and has a 65 amp output rather than the stock 35 amps. (top alternator is accord, bottom is stock)
To get the spacing right I just took a file to the back side of the front mounting tab. I used a pulley from an unknown ford alternator that a friend of mine had lying around. It has v-belt and a smaller fan. When installed, the fan interferes with the lower radiator hose, but all I had to do there was file down the tips of the fan blades until there was enough clearance. The last thing I did as far as fitment goes is put on a 1in longer belt. Pulley alignment is pretty much perfect and it fits perfectly in the stock mounting brackets.
For peace of mind and safety sake, I zip tied the lower radiator hose firmly in place so that it won't somehow come into contact with the fan since the clearance is pretty tight.
OK, so then there'e the wiring. I needed the connector for the alternator so I went to my local pick-n-pull and went ahead and grabbed the voltage regulator and it's connector too. It wires in place of the stock regulator almost identically. I just followed the wiring diagram for each vehicle and made an adapter plug for the new regulator. The big different is that the honda regulator needs to be "turned on" by having the charging lamp wire grounded through a resistor. In the honda, it's actually wired through the choke heater and then through a resistor to ground, but instead I used an 8ohm, 20watt power resistor from radio shack. Too much resistance and the regulator won't turn on. Too little resistance and it will over charge. The webber carb on my 620 has a choke heater that has 9 ohms of resistance which doesn't allow the regulator to turn on.
The only thing that is strange and I haven't figured out yet, is that the charge light in in the cab doesn't ever completely turn off. It just goes dim. Other than that it seems to work perfectly so far. The mounting that I have right now is a little messy, but it's secure for now.