Misfire on 3.9i v8 RRC

My 93 V8 is misfiring on cylinder one left bank. It is sparking OK at the plug. I am struggling to find the answer to my problem. I have recently identified the following problems and wouild be grateful for feedback on what effect they would have:

The vacumn advance unit does not hold pressure when the balck pipe is sucked, indicating the diaphram maybe split.

The exhaust is leaking from joint of the left bank manifold and the front (cat) pipe

thanks

vacuum advance or exhaust blow will not cause a missfire

It may be sparking when you pull the lead off and crank/run it with a plug in the end but is it when its under pressure? i.e. with the plug screwed in and lead on.

check compressions, make sure you're getting fuel to the cylinder in question, if all ok, replace the plugs and leads.

Its common for the leads to break down under load.