I've done some work with a couple of 270 wby and own one myself. You must have some freebore in that custom if you can shoot factory with no pressure signs?? Have you chrono'd the factory rounds??
If I had to recomend I'd say I7828 is the place to start. With 150's probably anywhere from about 70 grains to as much as 73 or 74--stop when you match the factory speed. If you find a real sweet grouper in that range probably stop there too cause 100 fps isn't worth much compared to super accuracy. My factory weatherby was a real average shooter with almost any bullet till I tried 140 grain a-frames. For some reason it will launch these at 3325 fps and group 3" round groups at 300 yds all day long. My friend has a custom Rifles
inc and it likes RE 25 with good old 150 grain hornady interlocks. (Not interbonds...interlocks the cheap $.15 cent bullet!!) You can load the RE 25 about 1.5 grains more than your favorite load of 7828 but again WORK UP. I just got some of the new 7828 super short cut powder but haven't worked with it yet. I believe that it would be a great choice for the 270 wby AND I am anxious to work with '06 sized cases with heavy bullets with the same powder.