Madbull receiver and ASG gearbox issues

[MasterGouken]

Anyone have experience installing an ASG Ultimate gearbox in a Madbull receiver? I am trying to do this but the gearbox doesn't seem to sit right.

If I install the gearbox into the lower and insert the rear body pin and center body pin the pistol grip doesn't go on all the way and the upper doesn't sit right.
If I install the pistol grip first the rear body pin doesn't line up (the hole for it in the gearbox is about 2mm too low).

Is this a known incompatibility? Could it be an off spec receiver? The gearbox fits fine in my G&P receiver and i've tried different pistol grips.