I load for an old Dutch Steyr rifle chambered in 6.5x53.5R in which I make cartridge cases from 303 Brit necked down to 6.5mm (.264 inch). Being an older rifle the pressures are kept low. 140 gr bullets are launched at about 2200 to 2300 fps with no pressure signs. I use 6.5 MS dies and starting load data to load for this cartridge.
The load data for 6.5 Dutch is available on
www.surplusrifle.com under the Stery/Mannlicher forum and is straighforward in application.
I have in the past read of the popularity of 303 wildcats in the Empire of which the 6.5/303 was one. However I cannot recall where to find the exact data for that cartridge. Therefore, you will want to compare 6.5 Dutch cartridge dimensional and pressure data carefully against the 6.5/303 dimensional and pressure data before using 6.5 Dutch data. This load data s will give you a starting point in your search.
I hope this information is useful to you.
I have lurked on this website ever since I found that John B. often posts here and have only yesterday signed up for real. When I found your question regarding the 6.5/303 I realized even I could possibly contribute information to a learned group such as is found here.
Good luck.