have a chance to buy a 58 ford o matic for my 55 but it has a fine spline output shaft.i used to hear there was a special u joint to use a latter yoke on the early driveshaft.or was this just a story.i hate to cut my shaft.

There are some crossover U-Joints out there where the width of the cross and/or the cup sizes will vary so that mismatched combinations can be put together.  I remember my 1962 Thunderbird having one of those oddball U-Joints as factory equipment.  When possible, I use transmission yokes and/or differential yokes that have the same U-Joint dimensions as the drive shaft to minimize using those ‘crossover’ U-Joints.  I’m fortunate to have a collection of those various yokes so it does minimize U-Joint fitment issues when I’m swapping out transmissions rear gears.
 
Once you know both the cross width and cup diameter, then someone with a U-Joint catalog should be able to get you the number of the particular U-Joint you need.

Lorena, Texas (South of Waco)

I changed my 56 Ford-O-Matic for a 1962 medium case CruiseOmatic and the output shafts were different and needed a new/different yoke. The shop making the new drive shaft supplied it. A good shop will have anything you need including the u-joints.
They had to do the rear end over when I replaced the stock rear with an Explorer that had a replacement standard style yoke instead of the flat 4 bolt plate.

56 Vic, B'Ville 200 MPH Club Member, So Cal.