Will I be at a disadvantage using them, or should I find some in 70mm cases?
Shooting technical advice
Excess chamber length is of no consequence when choosing cartridges. In fact, most clay shooting and game guns these days are constructed with 3-inch (76mm) chambers, yet will shoot cartridges of 70mm, 67mm and 65mm equally well.
If you find 65mm fibre-wadded cartridges are available at the right price, you will not be put to any disadvantage.
The important thing is never to fire a cartridge longer than the chamber length, with the exception that some loaded in 67mm cases are advertised as being suitable for 65mm chambers.
In this case, go by what it says on the box.
Incidentally, the length of a cartridge is the length of a fired case, with the crimp undone, so a 76mm cartridge will fit onto a 70mm chamber, and allow the gun to close.
However, firing such a combination is highly dangerous.