QUICK UPDATE !!
I pulled my rear axle out this afternoon and the spool fits perfectly
I fitted the bearing clamps and its nice and snug with no sideways play and ZERO resistance when gently turned with my finger !!
I stripped the crown wheel and re fitted the old diff cage so I could put a straight edge across the flange face and scribe some marks on the diff casing. Then re fitted the spool and did the same. The difference is 7.9mm so I need to skim 3mm off the back of the crown wheel and 5mm off the flange face of the spool. The spool flange is about 22mm thick so with 5mm removed it will still be 5mm thicker than the MGB diff flange.
I rang my mate at 2pm to see if he was still at the machine shop and he was just washing up to go home. So I flew down there and dropped the bits off to him so he can do the machining and re drill the spool on Monday / Tuesday next week. So before the end of next week I can have the spool and crown wheel fitted for final testing by putting some gear marking compound on for rotation / mesh checks. I think this may just be a formality because I should be able get the crown wheel fitted back on within +/- 5 thou / the thickness of the line the scriber made
All looking good so far. UPS says the drive shafts have been in transit from California for 36 hours now
Goodness knows when they will arrive but after weeks of messing around trying to get the ones I want I at least know they are on the way -- Hooray !!