I also think a full 7" would work. I even got thru 30 gal on my final batch this year and I have 5 sets of plates on it. I don't know which, but I've seen some with thicker hollow plates, they would give you more capacity. Mine is a Lapierre and I think the hollow plates are about 1" thick. One thing is certain, an aluminum press will heat up much faster than the cast iron one you now have, I would think an SS would heat faster too, but not as fast as an aluminum.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.