i think he wants the sub frame pbody,ya know quick ratio steering like most people do.i have several for sale,78-79 sub frames complete.The most popular sub frames for upgrading cars with drum brakes is the mustang II and the second gen camero/firebirds.
Those 68 tempest steer like a barge,drum brakes suck,and if he goes ahead and subs it now,maybe he can get parts for it later when it needs a center link or lower A-frames.