OK! Grynch,
To all intents and purposes I think you are on the wright track. I just thought there is no need to split the Insert if it has a fair bit of length.. Cut part of the top off the fury piece and slide it into the outer mast to find how much you can remove. Mind you the extra length should only stiffen the Base is all.. My masts are 100% CF therefore very strong but brittle and aren't forgiving. There is no warning, they just snap. if you use a meter and a half it would be good. The fury mast section is most likely just losing its top coat and so will take the resin well.
I would suggest that you check the taper of the Duo and the Fury one and if they are the same then it should work well if there is a little clearance at the top between the insert it won't hurt as the Duo will flex and come up against the inner however the resin will take up any slack or most of it anyway. Gut feeling/Common Sense are much the same you won't do any harm and will give the mast a chance to be forgiving in tough conditions.. I've pulled 2 down on myself through not doing the job as it should have been done But this last one was done as you are doing and it has had a recent bit of rough treatment and stood up to the test.

Ron

Yeah! Way to go Grynch. Looks like you nailed it right first time..

Like I said when you fit the turned block of wood you should work out the sides and about 25mm up from the bottom Countersink and screw it all in place. When my mast got hot the resin softened a little and the Reo piece started to slip under strain.. Just a though and it is your mast.
Keep us informed as this is good stuff for anyone else doing the same thing.
Ron

make a drop in for your yacht, and then have the ally slide over it.
keep in mind you will really have to look after this ally because if you dent it it will probably fail!!!!

Gizmo,
I would suggest one of two things and the first one will make your back terribly straight[}:)]. The other is a tube that is as close as you can get to it's internal size and around 3/4 Alloy parts length and fit it.. I don't think that any clearance is really going to matter as the outer will just curve until the iner takes up the slack and does its job then when the load comes off everything will just relax into its original shape.. Don't try to glass it all the way up, if anything you could glass band the top, bottom and middle with a 50 or 75 mm wide Glass strip to make the inner dia of the outer tube.
Just a though and you shouldn't have a problem with it..
Ron