I have a much neglected project with a 1992 - 4L60 (non E) that won't be using a ECM to activate the TCC 4th gear loc-up.

Since I won't be using a ECM and will be needing some assortment of parts (OEM & some aftermarket) is there a simple way or complicated way that replaces the ECM's signal to lockup in 4th gear only? A tried and true methiod would be GREAT as ive seen and read multiple ways to do it (Professional and Backyard ways), some with few parts and then some with more parts as well with much more $ dollars involved. ANY help or Links is Greatly Appreciated !
PS; The Vehicle is a 68 C10 with a SBC motor. Trans casting #'s 8676358, Suffix code 2FTM098H passenger side above pan location.

Both motor and trans are from Firebirds, 1988 305 motor and a 1992 4L60 trans - this is why I've posted here thinking I would get the best help.