The popping and backfire is still there. But it was worst right after start up and engine ran a bit smoother after a couple of minutes, still it had hesitation when giving throttle.

Since most tests I've performed has been ok, I will check once more for unmetered air and I will try another ecu.

My '77 had a loose connections from the EGR tube to the intake manifold and rotted a hole under the intake. It's a recall item according to a restoration book I have. Get a mirror and hold it under the end closet to the windshield and see if you have a hole. I hope you don't but it happened on mine.

I will certainly check it! Every thing I can rule out will get me one step closer to solving this mystery

Update on my problem.

It looks like I have found the source of the problem, although I have not been able to drive the car and feel the behaviour of the engine during load due to heavy winter weather.

But, after changing the spark plug wires the engine responded much better and no backfire of any kind. I did a recheck of the old wires and it turned out that some of the wires only gave spark every now and then. First time I started working on the engine I just checked for spark on one spark plug, should have checked them all I guess

Thanks for all help with my problem

Now I can move on and deal with the rust issues. Is the area around the hood hinge on the passenger side a common area for rust? The coolant expansion tank is close to the rusted area but haven't seen rust close to that on other cars...