For a while now, my 914 has had an issue with smoke entering the cabin. Here are all of the clues, can somebody help me figure out what is going on?

• Smoke comes in from the dash area - dash end vents, defrost vents, sometimes seems to just come from under/behind the dashboard
• It's not a forceful "blow" of smoke, more a "wafting" - but a lot of it when it happens.
• It seems somewhat related to revving the engine under load - rev to 4000 RPM or so and get 5-10 seconds of smoke. Doesn't seem to happen revving at a standstill (i.e. no load)
• I don't think it happens when the engine is cold... usually after 5-10 minutes of driving to warm up
• This has been going on for a long time (months)
• According to my mechanic's 5-gas analyzer, no CO is entering the cabin.
• Doesn't smell electrical. The odor is faint. But it's been bad enough that I've had a sore throat after a long drive
• It happens regardless of the position of the heat lever/flaps

At first, I thought it was electrical, but I'm fairly certain it's not - Amp gauge stays normal, all electrical components on the car work properly, and it doesn't smell like an electrical short. My next thought was that either there's an exhaust leak inside the SSHEs, or oil has dripped inside the HE shells and revving the engine heats it up enough to burn. But I'm skeptical of this because the problem seems totally unaffected by the position of the heat lever... would expect it to be worse when the heat lever is pulled, but it's not.

Another clue: Yesterday I revved in first gear pulling into a gas station. Smoke from the dash. Pulled the front trunk release to fill the tank and saw a bit of smoke wafting from under the front trunk, through the gap at the edge of the trunklid. So I'm starting to think it's HVAC related.

Final clue: I noticed yesterday that my charcoal canister (stock location for a '74, on top of the fuel tank on the driver's side) seems to weep a tiny amount of fuel. Thinking this is totally unrelated, but not certain.

Given the leaky charcoal canister, I might take this opportunity to pull the fuel tank (have never done so in 2+ years of ownership), then pull and clean out the HVAC box...

You have oil burning off somewhere in your heater boxes, possibly. Are they in good shape? Inspect that area real good. Kinda normal if car has been sitting a long time with rusted boxes, but you say you use yours often? Usually it will burn off real quick and be done. Feel on top of your heater boxes and see if there is oil on top of them and in channel. if they are rusted out then the smoke is getting pulled into your cabin, because the flappers are stuck cracked open or something.
Charcoal canister issue? Don't know why you would have fuel weeping unless you got a bad vacuum problem. Hope it is unrelated.
Good luck and let us know what you find.

Hypnosis, patch, gum?

Valve cover gasket is leaking onto HE

Yup

It also could be your push rod tube seals. When I fixed mine, the smoking stopped.