TBI did not start in the G-bodies until (1986 or) 1987. The Malibu in all body styles ceased production in 1983, leaving the Monte Carlo and El Camino through the 1988 model year. I tried looking up TBI stuff on 1978/88 El Camino, but RockAuto does not have much. Others with detailed knowledge should weigh in.
I purchased a 1987 Monte Carlo in-tank TBI fuel sender from GM to support putting a RamJet 350 in my coupe, but the wagon tank is a different shape. I recall in the thread listed above that the Forum member purchased an Astro van sender and it lined up pretty close. Another experiment could be to locate a full-size 1987+ Chevy truck fuel sender and see if that would go to the bottom of the wagon tank. Don't forget that these old school tanks had no fuel baffle at the bottom, to keep the fuel from running away from the pickup at low tank levels and hard turns/launches.