This applies to all transmissions that were originally used prior to the introduction of the LS platform around 1999. So your th350, th400, 700r, early 4l60e, early 4l80e, etc, will need to have one of these crank spacers. “crank spacer” is a bit of a misnomer. The only purpose of this spacer is to artificially extend the rear of the LS engine’s crankshaft. The reason for this is the rear of the LS crankshaft being 1/2″ shorter compared to the old style engines like a 350/454. The transmissions that come with the LS engines use a torque converter with a longer “snout” to make up this difference allowing for proper torque converter support in the back of the crankshaft. Failure to use one of these crankshaft spacers with an old style trans (th350/th400/etc) would result in a torque converter that is not properly centered which could cause serious damage.
Again there are different styles. One simply presses into the rear of the crankshaft. This style is super simple to install and extremely affordable. The other is a little more substantial and is a part originaly developed and used by GM themselves. This style bolts onto the crank using the flex plate bolts. IT requires the original flexplate bolts to be removed and discarded. Then you bolt the spacer on using new, longer bolts. These days the press in style seems the most popular. The bolt on style could be argued to be stronger and more reliable, but either are generally fine to use.