Re: Engine Help Please!
If it's a 350 with 400hp it likely has a pretty decent cam, which means you need to do some serious timing work, initial, curve, total, and vacuum advance.
Take the cap off, turn the motor to COMPRESSION stroke on the #1 cylinder, and post back where the distributor is pointing. It is VERY easy, even for us big time gear heads to be one tooth off, I bet it is one tooth retarded from what you posted.
If it's not, disconnect the vacuum advance, set initial timing to 16*-20*, re set idle speed, and go from there, don't worry about the vacuum advance if it's equipped until you figure this out, that's a whole different story.
Your total timing can still be correct, even though you are a tooth off.
One way to tell if you are a tooth retarded is advance the distributor A LOT and see if it idles any better, if it does, you are at least a tooth off.
The only Mustang I'd ever own is a Fender.
'68 333" Camaro
'14 5.0 Mustang