Hi Joe, welcome back!
Not all Electrums have the same size pickups or electronics setups, but I'm guessing your '83 has the wide casing pickups rather than the narrow casings. That said, I'm not sure but it looks like those are, if not actually AXY's, the same size as AXY's, which means that the Fatboy would be a direct replacement. The Fatboy has a wider aperture and therefore a fatter sound. However, I'm just guessing. I can't recall reading here before whether the Electrum pickups are AXY's or are the same size as AXY's, or how wide the aperture is. Nor do I recall anyone asking about dropping Fatboys into an Electrum.
Alembic pickups are designed to produce a clear representation of string vibration evenly across the frequency range. They don't color the sound in the way other pickup designs may focus on certain frequency ranges.
Because the physical vibration of the string varies across the length of the string, the tone reproduced by a pickup will vary depending on it's placement under the string; move it a quarter inch forward or back and the tone changes. Therefore a wider aperture will sound fatter bacause it sees a greater length and therefore tonal variety of the string.
Because of the clear and even reproduction of string vibration provided by Alembic pickups, the player then has a broad canvass upon which to create using his or her amp and other tools. Thus in discussing ways in which to get a fatter tone, it might be helpful for us to know what kind of rig you are playing through; your amp, your effects, etc.
Another thing you can do is change the heights of the pickups. Lowering pickup height will produce a cleaner sound; raising the height will produce a fatter sound.
