Hi Paul and
to the forum.
Dennis is correct, the MPIII axle is 14mm diameter with 10mm wide flat ends and the rear is designed to fit a standard 135mm wide dropout.
You could probably add some spacers to enable it to fit a maximum gap of ~145mm before you run out of sufficient thread to securely tighten the axle nuts correctly.
Unfortunately your 7" gap is going to be about an inch too wide for the MP's axle.
If it's a steel frame, you might be able to reduce the width of the dropouts by at least an inch by gently bending the frame together using some threaded rod and a couple of nuts, but you must not try to bend an aluminium frame unless you really don't care if it breaks!
Alan