Ok, so Im working from memory here... Its been 4 years since I sold stuff for a 90.
1. You are right in thinking you do NOT want the HD King Springs - in a usually empty they are WAY to hard.
2. In a usually empty 90 you do NOT want the 90 series Koni's these to are WAY to hard and combined you will not have any suspension or axle flex!
3.KRFR-03 or KRRR-02 are a good choice for the front - they are about the same but the 02's are a tad taller and can be used to level the normally lower front end up a bit if vehicles are normally empty.
4. Check the diameter of the rear springs.  From memory the coil diameters on a 90 are smaller than on a 110, therefore the KLRR-05's will have too big a diameter.  Someone else can come in and confirm this.  If my memory is failing me and the 90 & 110 rear springs are the same diameter then the KLRR-05 are the ones to get.
5. The rear propshaft will be fine with KLRR-05's
6. Trailing arm bushes at the chassis end will flog out A LITTLE sooner than with standard height springs, but this really isnt a problem.  The cure is cranked rear trailing arms, but it really is an expense that isnt necessary.
7. Steering Geometry - There are several ways of overcoming the castor issue.
7a. Front propshaft.  This should be changed for a double-cardon prop (not the disco unit and changed transfer flange if you want it to last).  Ive always found the new prop to be the best solution (Juddy on here can sort you out with an excellent unit from the UK)
7b. Front Radius arms - Ideally you would change these for either +3 or +6 degree ones.  You can save cash by removing your existing bushes and replacing with castor corrected ones, or by slotting your swivels.  My money always swaps the arms for +3 or +6.
So now, having shot your plans to bits 

 what dampers would I suggest... Difficult choice as there are hundreds to choose from. If money isnt so much of an issue, Koni 80's, on a bit more of a budget but always had very good results and customer feedback and work well with the King Springs, Tough Dog Foam Cells (not the adjustable version).  Bilstein and OME are also worth a look.
Make sure your dampers will compress to the same length as the ones that come off and extend approx 40-50mm longer than the standard ones.
hth
Jon
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