....It is my understanding that the seals are part of the shimming process on the tranny....
This depends on the age of your swing axle tranny. On early models (up to halfway through the 67 model year) the shims were also the seals. After this, the shims were just shims and an O-ring provided the seal. If you are leaving the tranny in place, you can replace the boots with the 'split' type. if you are removing it, then you have the option to install the original non-split type of boot.