A different transmission. If you don't want lock up, don't even bother with that 1988 unit. I've said this many times and 360duster knows - trying to make a 1978-up lock-up unit into a non lockup trans. is really difficult if not impossible because of the intricacies of particular parts involved and actual availability of said parts.
Unless you are over 600 hp (which I doubt with a 318 but hey, what do I know) or if the truck is real heavy, any garden-variety 904 variant should be fine as long as you build it with good parts. There are thin clutch/steel sets that you can get for more holding power. Billet spring pack and servos, 4.2 lever, good bands and struts, good valve body, done. Either A&A or Cope Racing should carry all this stuff.
