old_school,
May sound silly but I'll ask anyway.
If the gearbox is pointing down at the back, i'e engine + gearbox are sloping down towards the back of the car. IF this is the case the diff pinion must be pointing up. Remembering the angles are relative. So, to make the diff pinion account for acceleration you should mount it 2 deg twisted down from being parallel with the gearbox angle. Don't laugh, I've got this wrong in the past.

If your issue is tracked to you pinion angle, from what you have described below you have too much angle as the noise is evident under coast and decel (assuming the angle is in the correct direction). Under Decel the pinion is being twisted anticlockwise (Looking from the passenger side i.e forcing down) which will increase the angle difference. Under Accel it is being twisted clockwise and closes the angle difference (This is what Caltrac/traction rods try to stop, the upwards twisting of the diff which causes leaf spring bind/twisting).

Another issue I have seen with the way CAs mount into 1200 is generally you do as described above due to the space in the trans tunnel, this then pushes gearbox oil towards the rear section of the gearbox and it sometimes comes out past the rear oil seal under acceleration(Yoke area). As we all know, CA love to feed excessive oil to the heads and I have always had issues with the rear of the passenger rocker cover leaking (due to the rear slope of the engine and CA head oil feed).

I've made some pretty heavy changes to the trans tunnel to get the gearbox to sit higher, thereby decreasing the rearward slop.