When I referred to the High Range I was meaning the 2-4-6 speeds.  I  refer to the 1-3-5 speeds as being the low range.  The 2-4-6 speeds drive through a different gear arrangement than the 1-3-5 speeds.  The drive gear for the 2-4-6 speeds only engages about one-fourth of an inch using a male/female connection and I believe this is where your problem is. It has been over 50 years since I last replaced these gears in a 70 to correct the very problem you are experiencing.  If you have access to a parts book or better yet, if you can get to an Oliver Service Manual for the 70 you can better understand what I am saying. There is small roller bearing that you will want to replace along with both the male and female gears.