The 5 arm spider of the FC-M739-5 has a BCD of 94/58mm whereas the 5 arm spider of the FC-M950-HG uses 110/74mm. Is it possible to use the XTR spider on the XT crank?
Unless you are a super-capable machinist with incredible machinery, that's not possible. I'd say, practically impossible in this case.
If this is a life and death situation, I'd cut the old spider, fabricate a hollow disc that fits into this that is having 94/58 pattern, and bolt into the crank. This is a bit easier than machining a Hirsch joint, but still very, very hard even for an expert machinist.
I'd say machining a new crank from a block of aluminium is probably easier. Sorry.
Yes, it's a fit with XT.*
You can't interchange with LX or STX-RC though because, even though the spline is the same, the spider is thicker (maybe the material is softer).
You can borrow my lockring tool if you like though I think it's the same as the ISIS BB tool.
*you might need to file the chainring arms slightly to clear the crankarm when using the XT spider on the XTR arms. XTR spider fits XT no issue.