Big can of worms..

Where in Michigan??

First thing that caught my eye was the initial timing and the "heavy springs" I run a light spring and a medium spring in my 400's.

How long has the motor been acting like this?

Vacuum is good at 15" Nominal is 17"

I would check all cylinders with a compression tester and at same time with a vacuum gauge. You may have a few burnt valves, a valve not seating, leaking valve seats, worn guides..maybe all.

What I would do..address pinging. Siphon all gas from tank, buy 5 gallons of 110-116 octane and 5 gallons of fresh 93 octane for a base of 102-105 octane. That should cure the pinging issue in order to run vehicle time and calibrate properly (not to cure issue). A lean carb will induce pinging, so will a hot range plug. You have detonation at 12 degrees initial. but have to go to 8 for hot starts. Should not have to adj. timing at all once dialed in. Set initial timing. Advance timing should be 32-36 all in by 22-2500. Must adjust your springs to get all timing in by 22-2500 rpm. Heaviest springs I have never used in some 15 years. I do not like to screw around with a known "pinging problem" easiest solution is to raise the octane. If you have a 400 with 72 cc or smaller heads, you WILL be detonating under load with 92 octane, but will run fine at idle. Final CR for that motor, stock bore and 72 cc heads was advertised at 10.75 to 1 but in reality, around 10.3. Maximun CR with pump gas is 9.5 with a properly tuned carb, curved distributor and proper camming. Can go to 10 with real fine tuning.

Is cam original to motor? Has valvetrain been adjusted lately? any broken inner or outer valvesprings?

I say...base 102-105 octane. Check your compression and vacuum through rpm range, recurve distributor for initial of 8-14 with all in total of 32-36 all in by 22-2500. Check that carb is operating perfect..No hung up linkage, adj. bleed screws to optimize idle. (with vacuum gauge plugged in to large carb port) Check to see if secondaries open fully under full acceleration, and with that, the gas pedal linkage, when fully depressed, fully opens the secondaries. A good start for your carb bleed screws in close fully cw seat lightly. Open 3 full turns ccw.

Next step is to replace valvesprings. Can be done on motor. Or if desired, removed for complete overhaul. even though springs may be intact, they loose their tension over the years. While off, take to a shop for overhaul.

You could also have a dirty carb, bad gas, clogged fuel filters, cam lobe(s) wiped out. If rebuilt in 99, how many miles and for how many months was it sitting over those 5 years? Also if that pinging issue has not been resolved, there may be some internal damage with the main bearing clearances. What is your oil PSI at start up and when hot? Any fluctuation at idle? any fluctuation in neutral while revving from idle to say 4000 rpm?

Most 69 400's had 068 and 041 grinds. 335/366 HP. the 067 grind rated 350 hp. It is possible, but a longshot that the bird may be a 350..or a 400 with a 2 bbl. cam and the 2 bbl intake and carb swapped out for a 4 bbl and did not change the 2 bbl. cam and heads. Need the head and motor codes to reference. If you have done all the work, and know what you have done, you have covered some of the more detailed points. However, be cautious what other people do for you nad tell you. As Marvin Gaye once said...believe half of what you see and none of what you hear.

I am in Southgate. Would be willing to go through these issues if you can drop the car off for a few days to a week. We could probably address alot of issues while you are here with regards to the basics, but would need some time to get gears in head flowing.

I looked your post over again..There should be no evident pinging at idle (no load) with 92 octane. There has to be something somewhere not right. Possible dampner with timing tabs keyway broken causing erroneous timing reading, primaries too small or plugged up (lean), valvetrian geometry wrong (too high rocker ratio too small/big pushrods) Bent pushrods on intake side (caused by broken/weak valvesprings) I thought once that my 400 was having a detonation problem with 82 cc heads. Actually was 5 broken intake inner springs clicking inside valve cover which sounded like detonation) I ran the car at track in November, trailered it, then started it on a cold day in Feb. I broke 3 on one side, 2 on the other.