Stan, I think your fine with a 30". For giggles, I ran some numbers for you. The 30" barrel will yield higher speeds. But there are diminishing returns. The velocity begins to gain less speed around the 20"-22" length. But you still get a gain with each inch of barrel, that's common sense. The slower powders will help with that.
24" barrel with H4350 and 140 grain A-max ran with QuickLoad everything at 61,800K psi
2775 fps with 42.5 grains H4350
30" barrel- 2899 fps same powder charge
H4831 with 45.6 grains at 61,900 psi
24" - 2809 fps
30"- 2940 fps
H4831 shows a calculated 40 fps gain over H4350. It's worth a try to see what the rifle likes and see actual fps results. But still try the tired and true H4350. 40 fps, IMO, is close enough to a wash between the 2 powders. Now, if you went with a powder like R17, you would see more of a gain, but the temp sensitivity could be a big issue.
When I first started competing, I ran a 260 Rem with a 28" barrel. I was a velocity whore (at the time). I ran all the numbers with the various powders. My research showed that I could run H1000 around 49-50 grains and get the most speed out of my 260 Rem. The problem was, I couldn't get 49 grains in the case! It was a heavily compressed load, and the powder would push the bullet out after sitting over night. It would change my seating depth of the 140 VLD and would now be well into the lands. I couldn't get enough powder in there and keep the bullet where I wanted, so that plan was shot. I tried H4831 and H4350, and got a little more speed than H4350, but the H4350 shot MUCH better, was less finicky, and had a large sweet spot forward and back of my load, so that's what I ran. Let the rifle tell you what it wants. You will already be running faster than a common 6.5 CM, and will be flatter shooting with less wind drift. So, just run what the rifle likes, and don't worry about the speed.