John
I suspect you are right about the rates - its a place to start - I went for 2.25" shock bodies as the available spring rate range is much greater than the 1.9" bodies.
However, I want to challenge your math (for my own understanding).
If you have one inch of push rod / rocker movement and your rocker is 1.5:1 are you then moving the shock 1.5" or are you moving it 1/1.5 i.e. less travel which is implied by your suggestion a 400lb spring is giving you an effective 600lb?
1" of single wheel bump in my application is a little over 1/2" at the push rod (it is inboard and I have wide-ish track) which is why I am multiplying up.
Complicating factor is that 1" at both wheels in bump (i.e. both of them over a ridge) is 1" at the push rod and therefore 1.5" at the shock so I have to leave room for that two wheel bump possibility to have spare stroke in the shock and / or put bump stops on the axles.
I haven't got a clue about vehicle weight (unsprung) yet. I think I am adding way too much structural steel tube but if some is good more might be better - I will not need to add lead and it should be strong.
Assuming it was, for ease of math, 2000 lb and 50:50 weight distribution that's 1000 lb each end or 500 lb per wheel. With my bell crank leverage advantage that would need a 750 lb spring, less some pre-load by winding the shock collar up so I am guessing around 600 - 650 lb inch springs per corner in this scenario.
As an aside - I read somewhere with push rod / pull rod suspension adjust the ride height on the push rod, not the spring seat as that would alter the lever / bellcrank starting position and therefore the arc it travels potentially changing from rising rate to falling rate etc..
My scenario seems consistent with the rates you are proposing though you have more weight rearward than I am expecting - I will have a driveshaft at ~ 14" so my engine is a bit further forward - you are using a transaxle I think.
Anyone else with a similar setup please feel free to share you experiences here as its not too late to be told I have misunderstood