Anyone with any info on how much tapering shafts effects spine. I have some 23/64 Doug fir shafts that are supposed to be 65-70# spine. Most of them actually spine 70-75 on my tester. If I grind a 9-10" 23/64 to 5/16 taper on these shafts how much can I expect to drop spine. I normally shoot 60-65 spine cut to 27" from my 62# longbow.

1-3# is about it.  Most longbows will shoot overspined arrows quite well.  I think the heavier spines will work fine for you.

Could just leave them alittle long and try them at about 29".

Ceb, I did some testing with 11/32 and dropped a consistent 2 lb and 15 gr with a 9" taper.  With 23/54, you might loose another lb.  Removing wood from the ends of a shaft will reduce weight much more than spine; from the mid portion will reduce spine more than weight.  If you want to drop the spine, lightly sand the center part of the shaft and you can drop spine quickly.  Don't use coarser than 150 grit and check spine often until you get the hang of it; it can drop pretty fast.

2-3# has been my experience. Heavier points??

The static spine of the shaft won't drop much as noted above, but it changes how they react when leaving the bow.. similar to just using a smaller diameter shaft.  I went up 5# in spine to accomodate the taper (then went up another 5# for increasing my point weight) and kept everything in tune.

You may be fine with tapering, but just leaving 'em an inch longer or going up in point weight is a whole lot easier.