73 Duster going from manual to automatic

If that 400hp is with iron heads,then that will likely have a cam like the 292/509. And if you compression is not way up there then the take-off with stock parts will be sluggish.
So that usually leads to a hi-stall, bigger rear gears, and an overdrive. This means 2.76s is gonna suck with a 2000stall that the 318 car likely has; is all I'm saying.
But if you did your homework, then it won't be quite as bad as it sounds.
The trick is to have a reasonable amount of power in first gear at stall, and to have a lot of power when you nail it; For me that means 30 mph.

At KD into first (2.45 ratio), at 30 mph;
2.76s would be ~2650, so even a 2800TC is not gonna help you much.
4.10s would be ~3900, close to peak torque, so when you nail it,she will explode with tirespin.
3.55s would be ~3400, your compromise, and about as low as I would go with that 292 cam. But you're gonna need a bit of stall to go with that. The 318 TC is not gonna cut it.
Your results may vary.
An overdrive and 4.30s would be sweet, hitting 65 at about the top of second gear(1.45 ratio), ~5300, very close to the powerpeak. and she'll still cruise at 65=2400 in loc-up.(.69overdrive)

Just trying to be helpful; that 292 is not the most friendly cam I have used.