I have been shooting & loading the 264 since 1964. You need the slowest powders. I find H-1000 to fast a burning form the 264 except for the 100gr bullet. When it comes to 140gr class bullets Powders like H-US869, VV 24N41, Rel 33, Retumbo are the powders you want. I use VV 24N41 for 3222 fps in my 26" Bbl 264's. Not the slightest sign of excessive pressure. If you want specific info you can PM me.
^THIS^
Also, consider consulting a few reloading manuals closely before going any further with your development. What you are reporting doesn't line up, even closely, with any of the data I've compiled over the last 20 plus years. I've shot about every iteration of a 264win chamber I could dream up and have put together since 1995.
The data with the slower powders Hespco mentions can take some work to find. However, it is out there in bits and pieces and worth the investigation. 264 WIN greatly benefits with slower powders than most manufacturers publish data for it with, especially with longer barrels. It isn't a 26Nosler. Here are a couple of powders I've had success with over the years in several different 264win barrels and some ideas on how to utilize them best.
Retumbo is a good place to start with 140g bullets. Good case fill ratio. Start at 63g and if you haven't found what you are looking for by 68g forget it and move on, you will run into pressure some place around 71-72g with most 140g bullets in a SAAMI chamber but accuracy usually fades quickly above 68g. The down side of Retumbo, it can get spiky in cold weather with heavy charges, especially with faster twist barrels. There are several threads covering that so I won't waste time retracing that ground.
Norma 217 is another good choice. A step slower than Retumbo and slightly more bulky so it fills the case more completely. I usually start at 63-64g and work from there. The last four barrels I ran into pressure around 73-74g with 140g Berger hybrids and 143 ELDX and best SD usually happens at 66-68g. Burns noticeably cleaner toward the top portion of the safe curve.
PM me if you desire any further assistance, I don't share specific load data on the public forum.