You can use a 65-ton JagerMech mini as the 70-ton model.
And I'd suggest avoiding the JagerMech III mini altogether. It's a horrid, oversized lump. OTOH, the arms and legs from the JagerMech III (available from IWM) matched to the torso of a plastic JagerMech (from the old or new intro boxes) is a great thing, and easily fits as the 70-ton JM version.
And yes, the 70-tonner should have been called the JagerMech II, but that's not canon. See also Enforcer III - no Enforcer II, although the 5/8/5 mover arguably could claim that name.
W.