No, you can't use a steel as a pressure plte under the snap ring because the inside will distort and be pushed up loosing most contact with the clutch below except for the outer edge. You need to use a minimum of .110"-.120" milled pressure plate or foreward drum pressure plate from bottom that is pushed by piston which is about .120" around the snap ring edge but the rounded part needs milled .050" or else it will contact the front planetary ring gear when foreward clutches apply and reduce freeplay to the point of having none which binds the assembly. 6 foreward/6 high is easy to do though and you can use the thinner .060" steels in high drum with the .060" frictions IF the frictions have fluid evacuation "channels" in the faces so when the high clutches apply the fluid will be evacuated from between the clutches quicker(like tread on a tire on wet pavement vs a bald tire).
I have a set of Blue Raybestos clutches that are .060" with grooves. I got them at my local trans shop and had to wait a few days to get them in. I don't know of any other brands or types that are grooved except for the Blue Plates by Raybestos. I just remembered that around 6 or 7 years ago I ordered a set of foreward and direct Motorsport Blue performance clutches and the foreward clutches were also grooved but Ford Motorsport sold Raybesos clutches under the "Motorsport" name.