No matter which trans you decide to go with, you will need to at a minimum need to cut the floor/trans tunnel to install a four speed or the Passon overdrive- there are reproductions of the factory four speed "humps" that then get welded in place. Brewer's, AMD and others carry them. The rest- pedals and linkage- pretty much bolts in, most of the bracket and pivot locations are already present, you just need to come up with all the bits and pieces. "Welding in" the pivot must be referring to a couple of tack welds to hold the pivot reinforcement plate in position; the pivot itself does not normally get welded.
The Tremecs, however, require a substantial amount of surgery to the floor pan as well as removal and reconstruction of the transmission crossmember center section; and since this is a structural member and anchor point for the rear of the torsion bars it cannot be skimped on. If you're not comfortable with that level of modification, stick with the four speeds (comparatively minor mods as compared to the Tremecs) or as
@Inertia suggested have your automatic modified to full manual operation.
