[Spits] Spitfire Differentials

[Matthew Milkevitch]

Fellow Listers;
This afternoon I removed the differential from my '77 Spitfire, for it was
getting very noisy under load.  I have several used differentials available to
swap in (four actually), however I noticed that one of them is different than
the rest.  It has a drain plug on the bottom and it has 6 holes for the
mounting studs.  
I'm assuming that this differential is from an early Spitfire.  I have no idea
from what car it came from, since I acquired it through a large parts deal.
My question:  are all Spitfire differentials interchangeable?  I know that the
early units had different ratios, but will a an early unit mount up to my
'77?

Thanks for the information,
Matt Milkevitch'77 SpitfireWillow Grove, PA