I think I'd do some research on 5th overdrive transmissions. International & other mid-sized trucks came with them. Maybe GMC had them too. Also, it seems to me it'd be easier & cheaper to find a rear differential with ratio you need, or even change out the ring & pinion in your third member than to do the work of installing a 3 way in your truck.

1966 C-10 Short Stepside
HiPo
Step-by-step ~ and on-going resto build
in the Project Journals