Just to confuse the issue, I will mention that the timing is NOT adjustable on 86-on V8s. You are second in two weeks to say they did. I am not sure what is happening but spark is controlled by mapped calculation and a crank position sensor not distributor position.
My first mechanism for locating a single cylinder misfire is to use a 12mm open end wrench to loosen injector lines at the fuel distributor one line at a time (soak up fuel with a rag - not much will come out if done quickly). Feel the contribution of each cylinder as they are killed. This is similar to pulling plug wires without the shocking experience.
Common vac leak can occur down the back side of the motor where the vacuum source for the transmission modulator hooks to intake.
Bosch Master, ASE Master, L1
33 years MB technician