I have a special that I built and it has Protech shocks that utilise 1.9 id springs or 2.25id with an alloy insert.
The car is a sevenesque type using MX5 rear suspension and double wishbone front the chassis was designed to fit a Cyana or Fury bodyshell and came with 7" x 500 lb springs all corners. These were far too hard and I put 2.25 id 7" x 350lbs on the front and 2.25"id 8" x 250lbs on the rear which is not too bad however the rear was still too hard and I bought some 1.9id 8"x 175 lbs and after fitting them found the car was harder still!

A question why should 175lbs x 1.9 id be harder than 250 x 2.25.
It seems I will have to go down to 120 lbs and try that and bite the expense as I cannot think of any other way but trial and error.
What do other guys with indy rears use?

I think you should look at the 'shock' rates ie adjust dampers to their weakest.
also perhaps use lower tyre pressures.

175lbs shouldn't be harder than 250lbs spring regardless of their dimensions. Unless you are bottoming out on the bump stops.

Maybe, the spring was mismarked earlier.

Why don't you put 175x1.9 springs in the front and run 250 in the front to close it out?

Oh dear I am such a fool!
The adjusting screw was fouling the lower wishbone and locking up but thank goodness there is no permanent damage.
All I had to do was turn the shockers round and now I have a nice smooth ride and a car that can deal with our third world type tracks that pass as roads.
Thank you all for the helpful replies.