The 55 1st and 2nd along with earlier 235s' main bearings are pinned in the block. This makes it a real hassle should you ever have to change the mains with the engine in the truck. That said, you will have to address the longer and lower water pump on the 59 block. Which is worser? I've had to deal with both and I'd rather not deal with either but I think the fan issue is harder. Once you buy a set of main bearings you can forget about it, depending on how old you are. As Jerry said you have have either block machined to take the full-circle lip seal.
