29"? That's a long one, alright. Not sure what if any advantage could be wrung out of it over say a 26", but the powder of powders for any .270 we've ever loaded for has always been IMR4350. That said, given the length I'd probably be inclined to try working up using RL22, a bit slower burner. Never saw the need or was able to find any advantage in using any primer than a standard LR primer, a CCI200. My own .270 seemed fond of having a little starting room, about 60 thou off the lands which put the OAL at I think 3.330". Good luck. .270 is absolutely one of the all-time great calibers.
I can't agree that the, "powder of powders," for the 270 is 4350.
The 270 came to fame courtesy of Jack O'Connor's writings. And, about the same time, Hodgdon's became famous, largely due to the popularity of H4831 surplus powder. And Jack pushed, with a capital P, the use of H4831 powder in the 270.
Jack even credits himself with originating the long time, standard load for a 270 with 130 grain bullets, 60 grains of H4831.
Jack O'Connor wrote that he played around with the 270 and H4831 powder, as soon as Bruce Hodgdon gave him some to try. He said he found that 60 grains, which is the case full to the top, made an accurate, fast load. After he published this load, and wrote a few articles about it, Hodgdons gave printed loading information on it, using Jack's load, without any further testing on it, by Hodgdon's!
And here's something else. When Bruce Hodgdon first released surplus 4831 powder, they had no information on it, except they knew it was slower than 4350. Thus, they called the powder, "4350 data powder." That meant to just use it in place of 4350, because it was so low priced, and use the same loading data as 4350.
After Jack's experiments they determined a class for it, and named it H4831.
The term, a rifleman's rifle, has often been used to describe the pre 64 Model 70. Using the same terminology, I think it can safely be said that a rifleman's powder for a 270 has been H4831, ever since Jack O. made it popular.
All this to just say that H4831 should be great in your 270 with the 29 inch barrel. I hope you chronograph it, to see just what it will do with H4831.
To add something else. Jack didn't consider 60 grains with 130 grain bullets to be the end all load. He has printed in Outdoor Life, loads that had more than 60 grains in them, but most people couldn't get that much tapped into the 270 case and get a bullet pushed in.