3l swap, gains you HP and torque with a more reliable engine.
torsen LSD in the trans to eliminate the powder metal spider gears in the transmission
focus shift tower mod, to eliminate the bolt from shearing and taking out the transmission.
Reliability issues cannot always be modded away. An old plastic impeller water pump could already have failed and been changed out to the metal or better/newer plastic impeller models years ago. Engine wire harnesses might have insulation crumbling after decades, but replacement harnesses are old too. Replacement alternators are cheap to buy but hell to install. The stock differential is typically OK unless you treat your car like it's a racer.