they are strong boxes, only heard of your diff and one others on here in the last couple of days going ever!

i've got a mk1 w reg 2l focus with the mtx75 in, done 105,000 miles now and i bought it at 80 someithing thousand and abused it ever since including VERY hard launches at drag strips and banging it very hard through the gears!!! thats probably what finished off my leaky drive shafts

one thing i will say though is dont believe ford that the oil lasts a life time, no oil does so change it every so often. mine was changed at 74k when the clutch was done and i've done it again very recently and put in fully synthetic and the old red stuff was quite dark even though it had only done 30k. gear change is a little nicer now too 8)

as for using the later type box i'm 99% sure you can just swap the selectors in the boxes over. its on a recent thread somewhere.