Yes they are interchangeable, as a matter of fact I had a 1992 Grasshopper 612 which I mounted a snow blower off my brothers 618, they were a perfect match. I would suggest a set of tire chains plus you will need a wheel spacer kit for the chains to clear the frame. You will have to install extended lug studs to make up for the spacers.
Let me assure you that combination is a great snow blower, I moved some major drifts throughout the years. I ended up fabricating a set of drift cutters so I could cut through drifts twice the height of my blower with ease. The 612 had more than adequate power I might add, but on thing that I stumbled into after a few years I kept having trouble with my vac system plugging while bagging eventually finding out that my Briggs & Stratton wasn't turning anywhere near 3,600 RPM (2,950 RPM). With a little adjusting it made the vac/blower system work great. The by-product of this was the additional throw the snow blower had at proper RPM.
