rb20 auto and rb25 auto are the same so that is no difference. They may have different stall converter -- not sure there.
I think you will definitely need more than 3500 stall speed with that combo.. I would suggest around 4500rpm or possibly even larger. and cams to bring on power sooner. Rb20 needs it, especially with a turbo much larger than factory, where the small factory turbo is designed to give power down low to make up for capacity.
Factory stall on my rb20 R32 is 2500rpm (or close to), and that is shithouse even for a mild turbo setup which I have such as rb25 highflow.. Such a slug off the line, but when gets going it does mojo.
As for 3.6:1 diff ratio -- my thoughts are that will also be too small for that combo with the above done, and you'll be hitting the power band too late for you to get an optimal time. I would suggest an R32 4.3:1 (the factory r32 auto ratio), or even shorter for a drag setup. It seems like this is your main goal?
I would also go larger than 205/50R15's for tyres.. even if that means you have to run 16's to find the right width tyres (i suggest 2x GTR rims if u can fit them)
Will be interested in other people's thoughts on what is required.. the above is probably what I would do if building a proper drag rb20 combo ..but that isn't really my goal.