You have a Mercruiser set up now, right? Switching to a 4.3 means you have to worry about the two side motor mounts, the rest will be the same except for maybe a couple of extra mounting holes for the transom plates depending on the year and model outdrive you find in a donor.
All the inline to V motor switchers bolted up the inner and outer trasnsom parts, dropped the motor in and attached it to the rear motor mounts. Then while using an alignment bar, positioned the motor for proper alignment, made some measurements for the side mounts and yanked the motor out. They built the side motor mount support pads, bolted in the motor mounts, dropped the motor in again and did the alignment by raising and lowering the side motor mounts. The mounts on the transom are fixed, so only the front mounts are adjustable.
Hope that made sense, Arch.