Good morning.... wish I'd picked up on this yesterday, but I was out shooting :^) I'm honestly not sure how much help I'm going to be, as I have never shot that heavy a weight, and shoot with a 25" draw. That makes for a lot of difference in what works. However, never one to be bashful (or afraid of being wrong!) I'll stick my neck out and give a couple of reactions...
First, I can't comment on carbons. After some experimentation, i am convinced that carbons do not have spine. All they have is free will, and, generally, attitude. Enough said on that...
Given that length does soften arrow spine, the comments about being too stiff are probably on target, given your draw. This next comment is going to sound off topic, but it really isn't: Often we have trouble getting old antique sewing machines set up right.... finding the right needle and setting the tension. Some seem easy, others wilfully stubborn. The best course of action in the worst cases is to go all the way back to zero and start over. That may be your best approach here.
Deliberatlely go way down in spine... maybe 1916... go softer until you clearly start to show it in impact point and nock kick. Then work back up again. Way back when, I figured spine was going to be a real problem for me because of my short draw, so I bought three arrows of each of Easton's weights from 1616 to 2016, then threw in 2018 and 2117. I had them all cut to 28" (I never vary my arrow length) and left them bareshaft. When I get a new bow it doesn't take long to find out what flies best.
That worked well for me, so I went a step further... I ordered a dozen made up arrows each in 1616, 1716, 1816 and 1916. These were the sizes that were working out best in most of the bows I was getting. I was shooting right around 50#, a few bows higher and a few lower. Almost all of these bows where Hills (about 40 of them over time) but a few were old shoot off the hand longbows of about the same weight, but which took softer spines.
Spending all that money on arrows, about the cost of a bow, really was worth it. I now have test arrows for new bows, and can immediately upon completing testing grab a quiver full of something that's going to work well and go shooting. 1916 worked for most of my Hills at my best shooting weight. For wood arrows, I would simply match the spine that the best shooting aluminum tested at on my spine meter.
Now, I'm down in weight to 35# and finding that the 1816 and 1716 are working best for the new bows I am acquiring at that weight. The 1816's are flyling great out of the recurves I'm playing with and the 1716's out of the non-center cut longbows.
One thing I noted is that generally, in spite of the fact that everyone said to shoot "soft", i.e. about 5# lower than bow weight, I often got good performance on my Hills with as much as 5# over bow weight. I know Hill shooters who shoot 10# over. My heaviest bows, around 60#, did well with 2018's. I strongly suspect this had more to do with form than the bows.
I'd play with the nock point, too... up and down... Again, move it until you can clearly see "That's really wrong!", then start back the other direction.
One word of encouragement... it can be done... at least with woods and aluminums. I even got a beautiful match on a Hill that I had built with no shelf at all! Shot it like an English Longbow
One more thing occurred to me. You are moving multiple variables. Settle on one set of criteria... one tip weight, one arrow length, one arrow material (alum. for choice). Vary only one element at a time and study results. Changing several things at once leaves you with no new knowledge.
Now, finally, fer cryin' out loud, don't listen to me... I shoot 35# at a 25" draw... what do I know?
Captain Dick in Seattle
Who is getting mighty hopeful that his new Hill LBC will arrive late this week or maybe next week for sure, and figures it will shoot a 1716 arrow perfectly right out of the box. I'll let you know if I'm right...