Sorry I don't know much about those transmissions, other than I've only ever heard of them being used in the 3400 and 4400 utility tractors. I've heard of the 6x4 manual reversing and 4x4 auto reversing more often, and both of those I've heard of being in the utility (3400 & 4400) and industrial (3500 & 4500) tractors. I think that the 4x4 manuals were the lowest cost trans that Ford offered that was good for loader work at that time. I don't think that they were any less durable or more problematic than the 6x4, I think they just offered it as a lower cost alternative for folks who wanted a factory loader tractor and didn't have a lot of money.