I have not built this boat, however 3 ducks and a 17lt, I angled my cloth as much as possible. This kind of gets you on the bias, the cloth lays down better. Also the extra pieces are big triangles that are almost as wide as the original cloth. You might end up with a seam at the cockpit on the deck. At this location there is very little deck actually showing under the combing, and sand out easily.

Lay out the glass cloth.  Then spend up to 30 minutes smoothing the cloth with clean, dry hands.  Work  and work out every single ripple in the cloth.  As you work the cloth the threads of the woof  and warp of the weave will slide and shift and conform to the hull shape.  In areas that you thought you would have to make a scissor cut you will find that you not have too.  It is a heck of a lot easier to smooth out dry cloth versus wet cloth.  As mentioned above,  laying out the cloth on the bias, 45 degrees to the weave, makes it easier to get the  threads of the weave to slide and conform to the shape.

I glassed  two fins for a Kaholo , cut the cloth on the bias and was able to form a sock out of the cloth and glass the fins without a scissor cut or seam!

Many thanks for the good advice.

My key concern is getting two strips of cloth for the bottom and hull from a piece 18' by 51 " 

I measured the hull crossection length  at the widest point and it was 30" (no overlap).

the bottom panels crossectional length was 19 " (no overlap).

The instructions suggest some over lap. 

I am looking for some clarification or assurances on how and to what dimensions to cut the two rectangles. 

Should I lay out the first layer, smooth and then trim and hope the remainder is wide enough for the 2nd layer?

Un fortunately for these size pieces cutting on the bias is not a practical option. 

Thanks again