Lock-up on a 350 will give you about a 200 rpm drop....not really much good except on a long road trip, fuel mileage-wise.
In order for the LU to work, it needs 12v & ground at the solenoid, an rpm signal, and vehicle speed signal. With the different distributor, you probably no longer have a valid engine rpm input.
You can wire the trans with a 12v source, and use the pressure switch that is already there to provide the ground, but it will go into LU as soon as it hits 3rd gear. In order to control WHEN lu comes in, you can install a switch on the 12v power wire. Flip it on when you want LU to engage, and off when you don't. With the pressure switch, it will only engage in 3rd gear anyway.
Some folks use a vacuum switch to control the power to the solenoid, so that is also an option.
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