i do not believe automatics have synchros
a few of the obvious things that have to be upgraded are (more)clutches, band, sprag, etc...
heres a list of the buildsheet for my (used to be mine

) built transmission if it helps at all...
-Borg Warner Deluxe Overhaul Kit
-Alto Red Eagle 2.625" Wide Hi-perf. Band
-New GM 10-vane front pump, teflon bushing
-HD "The Beast" Sun Gear Shell
-.500" Boost Valve (higher line pressure)
-Borg Warner 29 element forward sprag
-7 clutch frictions in 3-4 pack
-Sonnax servo release check valve
-ALL sealing rings, Torrington bearings,
bushings, thrust washers, seals and
gaskets replaced. New filter
-New #716 Input Clutch Housing
-New TCC/internal wiring harness
-New GM Reverse/Input Drum
-Borg Warner Low/Reverse Roller Assembly
-Hardened Vane Pump Rings
-Reverse Input Piston modified
-Longest GM band apply pin, for minimum
servo travel
-Bonded forward/3-4/reverse pistons
-Spacer plate feed holes drilled, HD 1-2
accumulator spring. 3-4 accumulator
blocked.
All clutch packs set for minimum clearance,
electrical solenoids & manifold pressure
switch known to be good (58K mi. on them).
All hard parts hand-selected or new.