If ou know someone with a surface grinder I beleive you can have the centre diff modded from the disco to selectable, or if you want to bdge you can angle grind it, but I wouldnt want to do that on a permanent job.
T'other thing you need to be aware of is that the '77 and '380 box's are a diffo length to the series so you'd need to have the propshafts modded too.