Really, while a roll crimp in most cases is shorter than a fold crimp, in slugs I often see roll crimps much longer than in shotshells. What is the length of the crimp? That is, the distance between the front end of the shell and the front end of the bullet? If, as I suspect, it is about 10 mm, then a following option is available: unfold the crimp (that'll be easier if the end of the case is heated SLIGHTLY using a SAFE source, e.g., holding the shell against a light bulb), trim the case to 65 mm and roll-crimp it back. steve white above is right about the significance of the crimp for the quality of the shot, but I think in this case a crimp of 2.5 mm (5 mm / 2) will be quite enough for a quality shot, and if it results in slightly lesser pressure - so much the better for the old gun! In fact, the pressure developed by modern 70 mm shells (especially if loaded to American specifications) would be a bigger concern to me than the length of the shell. A still better option for you could be to research a safe load for your drilling (preferably using the same powder your shells are loaded with), disassemble the shells and reload them with a safe charge of powder into trimmed shells. That's a lot of bother, but, in light of the scarcity of prewar drillings that you've noted...
