I found out the king pins are real bad on the 71 521 so I am off to look at another parts truck tomorrow. If the King pins are bad on that truck I will just find another 720 to buy.
Originally I wanted to use a 521 front section of the chassis, long enough to incorporate the rear cab mounts plus 12 inches. Then take a middle section from the 67 520 chassis that incoporates the middle and the back cab mounts. Turn this one back to back with the front frame section leaving about 3 to 4 inches in between cab mounts. This is to ensure I have enough room to mount the upper 3 point seat belt mount sercurly. Then from a third frame (520) cut the bed section off 4 inches in front of the rear cab mount. By doing this I hope to use the factory 520 leaf sping mounts. This basically shows how the cabs will be placed back to back.