Let's take this apart.
"Improved" technically meant a factory cartridge was "improved" (usually shoulder blown out for more powder capacity) and you could fire factory ammo in the chamber to get "improved brass." Since there has never been a factory 6.5/'06 cartridge, then this factory round cannot be "improved." Rather, you have a "wildcat" that has to be "made." AT THE SAME TIME, in English, words get "adapted" widely and the term "improved" has grown beyond this strict sense... I'm bored and quit semantics at this time.
b). One of the reasons the 6.5 has not been widely popular is the 6.8/'06 better known as the .270 Winchester. When you can buy a .277 caliber rifle from a factory and shoot factory ammo (fairly widely available) why bother with something that uses a .264 bullet? At one time, Uncle Sam left fired brass laying for anyone who cared to to pick up and a trip thru a .270 full length die... !!
c). How hard do you wish to work? Mr. Ackley did a 2 volume set of books that cover thousands of cartridges, factory, improved, and wildcat... available from eabco.com, sinclair International, ebay sometimes... Several versions are in there with load data for powders from Mr. A's era, pre 1985. There are other authors... but Mr. A was out front and summarized the work of others, his era.
d). The old standard for many "imps" was to start with max factory loads and go up. Pressure dropped from larger powder capacity so this was fairly safe. YOU DO THIS AT YOUR OWN RISK!!! Me? I would start with 6.8/'06 cases (.270 Winchester)... You will need to neck the neck down in some sort of 6.5 die. .264 WCF Mag would have lots of clearance, but .260 might work.
Now you have to make a judgement call. You want to try a load from .270 or a load from .25/'06? Pick one for the weight of bullet (6.5 bullet obviously) you are using. Reduce it 5% maybe... Load the case. Put in gun tied to tire. Fire with string from behind barricade. Overly safe? I don't think you can be overly safe. !!
Read the fired case. Load probably way light... If not, figure out why first...
As said, 3 fired cases and many die makers will make you custom dies. (Not cheap, no telling what you might find on ebay???) ch4d.com, LEE sometimes, even Huntington.com the RCBS custom shop...
Then enjoy your investment. It will be a substantial investment, but approach the .264 Winchester magnum with much better barrel life. Well, somewhat better barrel life. Not for prairie dog towns unless you like buying barrels. Luck.