Swap non-lockup trans for lockup?

[57loboy]

I searched and read this recent thread (Lock up or non lock up.???? What should I get.) and didn't want to hijack it with my question...

I have an '87 GN that is an ex-drag car that, according to the prior owner has a transmission with a converter that is way to loose for the street and is non-lock up. It does have an automatic valve body. He doesn't recall the stall speed of the current converter. The old owner spent serious $$ getting this car set up, but as he last used it for racing 4-5 years ago, he's forgotten a number of details of what's what.

I am putting this car back to street use and it will see highway driving from time to time. I am trying to decide whether I should swap in a stock BRF trans from my donor car that is fine or whether I should try to make the current trans more streetable.

I know I can swap the torque converter out, but what would be involved to get the current unit back to a lockup model? Thanks!