I don't know for sure, but if you figure that plenty of MN-12's were optioned with 5.0's which is a 302, which also has the similar external dimensions as your 351w, it should be pretty straight forward. Also, the 3.8 is nothing but a 5.0 with smaller bores, longer stroke and the front two cylinders chopped off. From everything I've read, you can drop a 5.0 directly in place of the 3.8 thanks to the same mount pattern and bellhousing bolt pattern. The 351w has a taller overall deck height, but it should still drop right in using the 3.8 mounts. I'd definitely replace the old fluid mounts with solid rubber ones while you had the engine out, being that your going to add quite a bit more torque and the stock 3.8's easily rip the fluid mounts when racing.
A few regulars in here have done this swap. They will be able to verify this for sure. Try posing in the 5.0 forum if they don't see you here.