This one?

I can understand your frustration with such a shortish craft as this'n, trying to make 3/4" twist up & around so much. And the 1-1/4" width faces up eh?

Ripping those'll make 'em more flexible, yes, but at the expense of some strength given the loss to saw kerf. Instead, and if you have the time, you might try soaking those outwale pieces for a few days (well-wetted first with really hot water then wrapped in soaked-thru towels, the whole then wrapped in plastic sheet) then, once unwrapped, slowly bringing them up to fit before clamping well to dry in place.

Steam-bending'd be my choice but that takes some extra effort to rig up a steaming box.... Has some risk too, steam does. Can be off-putting to those unacquainted with how quickly exposed skin can get burned.  

Grand Banks,

The assembly wiki shows the rubrail installation process. There, Andrew makes up a laminated 2-layer rubrail for each side with no ripping, steaming or soaking. The rails are supposed to be oriented as follows:

which I believe is how you got the one to fit (good job dry fitting first, BTW).

If the manual shows the following picture and you were using that to set the orientation, note that he has already glued the 2 strips together here and what you're seeing is the combined thickness.

Since each piece of wood is flexible enough to fit and the glue is still wet, the pieces just slide around each other to achieve the bend. Try clicking on the picture to enlarge it (maybe right-clicking to view image, too) and you'll see the glue line running vertically.

Laszlo

Good info Laszlo, salvation's in the details.

And having ENOUGH clamps about when you need 'em!

 So thanks for the feed back guys. I had spinach for dinner, went to bed and tried again the next morning.  I did have success bending the wood in the correct orientation. 

It took a bit of experimenting.  I started with two spring clamps bracketing a C clamp at midship.  Then I used small bar clamps to snug in the mahogany at the transomes.  Then, I went back started adding clamps from the center out that allowed me to work on getting the board even with the top plant.  By having the ends clamped I now really only had concern myself with applying downward pressure not down AND IN.  That is much easier. 

I did this as a dry fit and left it for short period of time.  When I removed the clamps the board obviously sprung back but I think it did strech the wood because after applying epoxy it was much easier to maniputate as needed.  It actually came our really well and I am quite pleased.