Re: 97 Mercury Grand Marquis LS
I have done several "J-Mods". The modifications vary from year to year with different results. Carefully consider exactly how you want your transmission to perform, or else you will find yourself buying a new separator plate.
I have discovered, through trial and error, that the accumulator springs listed on the J-mod sites are just a starting point. Unless you are lucky, plan on dropping the valve body several times to get the accumulator spring rates just right. For instance, on my '05 4R70W, I found that just removing the inside spring on my 1-2 accumulator and installing the softest 2-3 spring (instead of leaving the 2-3 spring out altogether) works best my '05 CV Sport, for my type of driving. Additionally, I did not drill the separator plate for more flow to the accumulators, the '05 plate already has larger holes, I only drilled to allow additional fluid flow.
Use Mercon V synthetic fluid. It's expensive, so be prepared to save it, you might have to remove the pan and valve body several times to get everything just right.
The mods are worthwhile. My car now will just chirp the tires during WOT shifts, but shifts smoothly and quicker than stock, during normal driving.