Slow 1/8 with 74 440 dart

I read everything in this thread and the more I think about it, the more is everything pointing to the timing, or ignition problems being the issue. Are you sure that your balancer is actually correct, that the ring has not slipped. The 7" of vacuum at idle makes no sense, the 30 degrees at idle with 36 total make no sense. The engine making no low end power and no top end power make no sense. With a 244 @ 50 cam you should at least make some top end power if nothing else. Being that this is a relatively low weight A body with 4.56 gears and a 3000 stall converter it should get off the line pretty quick. I have run 2.0-2.2 60 foot times with stock 5.0 5 speed Mustangs in the late 80's with street tires and 3.08 gears... 7" of idle vacuum to me indicate low idle timing, not 30 degrees. That engine should easily pull over 10" of vacuum at like 20-24 degrees initial. What is strange is that you say that it was pinging at 4000 + which would mean that the timing is way advanced. You can easily lock out the timing on the distributor and just run it at 36 degrees fixed timing to see how it runs. Also check your ECU box to make sure that it is working correctly, and get a positive piston stop and check that your balancer is correct.

What I think may be the issue is that you have a stock distributor that has a ton of mechanical advance, like close to 30 degrees with some heavy springs in it that will not let it fully advance till 4500 + rpm or something like that, a smog era distributor. If you have a distributor that is like that, that engine will always be a dog. For it to idle you are having to maybe set it at 15 degrees or so, but then you end up with 45 degrees total. 15 degrees is too low to get your vacuum into the 10+ range and 45 degrees + will cause detonation and kill the power up top. Since the timing curve is so slow it will also never make any power in the midrange as the timing is just to little there. If your balancer ring has slipped it will compound everything as there you have no idea where you really are.

I would like to set the distributor with about 10-14 degrees of mechanical advance all the way in by 2200-2500 rpm, and set your initial timing at 22-24 degrees. If not a completely locked out distributor at 36 degrees timing. Make sure that if you have a stock distributor that the vacuum canister is attached to the distributor so that the magnetic pickup is not flopping all over the place in there on a loose plate.