1965t5, there is an easy way to do it the way you describe in the initial post. Not sure what year model truck you have, but here is how it works on my '57 3600.
Leave your voltage regulator where it is on the firewall. Use it as a dummy and a terminal block. Disconnect the ARM and FIELD wires at the regulator. You will not be using those terminals. Re-purpose one of the generator wires that ran to the voltage regulator by making it the 'one wire' for your new alternator and connecting it to the BAT terminal on your old voltage regulator. Keep the battery feed and the gauge feed on the BAT terminal (no change). Your firewall mounted voltage regulator will no longer function, but will still be there. The BAT terminal is now simply a junction terminal.
If you opt to use a three wire alternator (less expensive and better voltage reading), the installation is almost as easy. You'll need a diode in the ignition wire, but you can wire one in yourself with a Radio Shack diode or you can buy a plug that includes the diode for about $10 on eBay. Wire as described above, but also connect the remote voltage sensing wire to the BAT terminal of the existing voltage regulator. This will allow the internal voltage regulator on the alternator to operate using a voltage reading from the load at the junction terminal instead of at the alternator. Wire the ignition wire (the one that requires the diode) to the ignition switch side of the ignition resistor on the firewall. Very easy. Don't be afraid of a three wire thinking it is a more difficult installation. The remote sensing wire will give you better performance when lots of accessories are operating.
Matt
