This is just my experience talking here....Ted, BlownT, Putz, Ron or Mango will probably chime in here....
A kick-down linkage kicks the trans down into a lower gear for passing. Great for heavy cars....as light as a bucket is....I, myself personally, don't have one hooked up on mine. I had a blown 392 Hemi, over 600 horses, and when I stepped on it, it went. Now, maybe if you were running everything stock, I mean completely stock and had 2.90's or 3.08's in the rear, but these little rods usually only weigh about 1500....so I'd say they'd still be pretty peppy.
As for the vacuum modulator, if you want the trans to shift automatically, yes, you need one.
Without one, a stock trans won't shift. Now a modified trans without a Modulator will bark the tires on a shift from 1st to 2nd and sometimes from 2nd to 3rd.
The Performance shift kits and trans rebuild kits have a plug so you can do the track, street/strip thing....but for the street, I feel you need a modulator, a adjustable one.
I have just recieved my new aluminum block for my Inline6 cylinder. My Crank came in last week along with my rods and oilpan. My block is for a 4.625" bore and my crank is a 4.5" stroke. Since I'm only turning it to 5000 rpm and running a 4-71 straight up drive wise, the balancing guys were really sweating it. The block casting is max'ed out at a 4.5 stroke crank....they had to really cut in there to get clearence on the oil pan rails.
I'm running essentially a 2800 stall with a stock turbo 400 with a B&M performance trans-kit with a performance valvebody. With the I6 being blown and a big inch one, the torque curve should be really flat. I hope to bark the tires for every shift....It will probably be very squirrelly when wet.