As someone who communicates regularly with Joe at the moment happy to help...if I can.
Can I as what the ETT and ST angle is so I can understand the reach/rear length in context?
At the front you should have no fear around the 62-62.5deg mark.
I am at 61deg this is true, as is Chris,but his is 29 front only, not both ends. I would also add tho that the steering gets fast at that point (which is part of the point, but with a 51mm offset 29er fork potentially too quick on initiation, especially with 27.5 wheels. (I'd like a 44mm offset 40 but only way to do that is machine new crowns) however I'm used to it now and don't want to give up the 40's!
So if you do go to 62.5 or lower perhaps arrange to try a 44mm offset CSU (uppers). Ask Chris about the mod to increase neg chamber size at the same time. He may not suggest it as he has balanced the Murmur setup but worth asking.
You may not need to go as far as 460mm CS if the SA is right and you deliver the reach that way, 450mm should be fine, 455mm more than fine.
-38mm bb drop? My personal view (and it's just that) is that that is too low, assuming it results in a 332mm BB height ((740mm wheel dia/2)-38mm) when you go slacker, particularly with 29 wheels
I find it makes the bike less agile in fast turns when changing direction quickly. It's not needed for stability at this length and HA and you also don't get the weight transfer when you are braking on anything except steep stuff which can result in push (even at 64.5) this is what I am experiencing on my G13 (at 64.5 and lower)
You are already in the bike (it's long, thats the point). So I would suggest a higher BB too.
You could try that now on the current bike to see if it works for you.
I'd add a -2 headset and then 2 suspension bushes to steepen it and check the angles that should steepen your seat tube, increase your reach and lift the BB by about 10mm but you will need to measure it) (Chris has lots of bushes) but still result in perhaps a -1 deg slacker front.Ideally I'd say 445mm to 350mm BB for the 29.
Nothing to lose and pretty low cost for those little mods.
Alternatively I am picking up my G16 this week and that can run 29 wheels. It will have a 520mm reach, 660mm ETT, 61deg - 62 HA with 29 wheels, approx 350mm BB and I can set the chain stay to 450mm, 77 SA. You are welcome try it in that form. unless you are in Scotland in which case that's harder for me!