You will probably want a numically lower rear end. The gassers typically have a much larger rear ratio than the diesel vehicles. The rear end from an 80's Oldsmobile diesel should be easily got. Yes you will want a diesel torque converter. While turbos are nice, they make the install a bit more difficult. On trucks, the air conditionioning system usually have to be removed to get a turbo into the engine compartment. On a car these issues may be compounded. Saw a 6.5 installed in an early 80's Cadillac. It fit, but just barely, and I would have hatted to try and get a turbo into that engine compartment. He made a custom transmission crossover with his conversion, and had a US gear overdrive.
83 C10 Suburban, Silverado, 6.2, 700R4, 3.73, 31-10.50R15
82 C30 Crew Cab dually, 6.2, Banks, Th400, 4.10, Gear Vendors, 235-85R16
83 C20 Suburban, Sierra Classic, 6.2, 700R4, 4.10, custom paint, 285-50R20
95 Yukon, 6.5, 4L80E, 3.42, 265-50R20
73 GMC Astro 95, 8V71, 10 speed Roadranger, 110" WB, 6 each 11R24.5