So my question doesn't specifically apply to my thirdgen, but I figure it's relevant for those doing a swap.

I recently swapped a Holley in place of a Q-jet on a factory turbo 400 car. When I swapped on the new carb, I didn't think about the kick down switch.

Would you use a linkage operated switch like the factory set up or a vacuum operated switch like the adjustable B&M unit?

Why?

I would use the linkage operated set-up as you know it will kick in when you need it (directly tied to the throttle) as opposed to one that works on vacuum.

I have a TH350 in mine, granted the kickdown isn't hooked up, but I plan to use the linkage set up.

The TH400 is an electrical kickdown. Adjust it to kickdown only with a lot of throttle (3/4 throttle), unless you want an easier kickdown.