cut all your 6 foot pieces at 42",this will leave 30 " left over
then rip all of them down the middle of the 5" face...this leaves 2 pieces at almost 2 1/2" wide(of course)
now you get all the 42 " pieces together,and same thing for the 30" pieces.
the longer pieces are long enough for neck throughs,and the shorter ones for bolt on and set neck,they will have enough room for headstocks as well.
but see now,here is the strange part.you have the wrong size of stock optimally(1/2"),because for a six string you really need a total width of 2 1/4" at least, but you can do a couple of things...one of which i like better than the other.
the one i do not really prefer the thought of.
take 4 of those pieces with complimentary opposing grain,and glue them side by side(2 1/2" faces glued together) with 1/8" laminates of another wood in between all of them(possibly ebony?maple?depends on the color contrast you want).this makes a 7 piece laminate neck,which can be very nice if done with aesthetics in mind.you may have to adjust the thickness of the 1/8" strips if your wood faces being glued together are not already planed flat.
here is the thought i like best.buy some 8/4 (1 1/2")flamed rock maple,and some 1/4" bocote(all 2 1/2" wide faces,like the other wood) and use the flame maple in the center,with the mahogany on the outside(opposing grains on the mahogany now)with the 1/4" bocote pieces in between all of them.this is what i would do
presto...5 piece gorgeous neck blank ready to cut,thick enough for a neck through with no angle and about a seven degree headstoock tilt(depending on whether the headstock is six inline or 3+3)
or you could do both...and mix and match laminates as you wish to make different contrast and such.by itself,that mahogany you have is completely the wrong size(1/2"),but with other woods for lamiates,it would be great.
there re alot of things you will have to think through before cutting.but hopefully i have given you some ideas to springboard off of.
by the way,turning all the woods sideways like this and glueing the faces together makes the grain run in effect quartersawn...and this is how i have made ALL of my necks,except i use 4/4 lumber and make 3 piece necks
http://fullservesite.com/westhemann/neck%2...haping_0002.jpg
http://fullservesite.com/westhemann/neck%2...haping_0003.jpg
http://fullservesite.com/westhemann/neck%2...haping_0005.jpg
http://fullservesite.com/westhemann/neck%2...%20rod_0005.jpg
http://fullservesite.com/westhemann/carved...%20set_0004.jpg
can you see what i mean through these pics?on the last pic the neck is not all the way visible,but you can JUST make out that it is a 3 piece