The critical measurement is the distance from the case head to the neck/shoulder junction. The Lee Collet has a tube on the inside, the bottom of the tube contacts the case holder and raises up into the die until the splits at the top of the tube are squeezed onto the mandrel by a constriction on the inside of the die body. The tube is one piece. You can not change the length of the tube.
However, you can fool the Lee Collet by inserting a washer on top of the shell holder
Now the critical case-head-to-neck/shoulder dimensions are:
6.5-06 is 2.1730"
260 rem is 1.7797"
for a difference of .3933" which is about 4 washers worth. IOW you could thread your 260 rem Lee Collet in your press, insert a 6.5-06 case in the shell holder and put 4 washer of ~.10" thickness each around the case and it would move the collets up to the neck of the 6.5-06 case.
Unless my math is wrong.