In my experience OME springs offer a good choice of options regarding spring rate and ride height , however the coating on them is not durable when used in UK conditions and flakes off quickly .
I will see what springs I run currently as my 80 is in a similar spec to yours for a lot of the time. I have a 2 inch lift.
+250 Kg on the rears would be about right , don't forget you have lost a lot of weight compared to a stock 80 by removing the rear set or sets of seats and your platform and draws probably is lighter than seats .
Julian probably offers the ultimate set up with his progressive +3 inch Slinky Kit ....that will be my next set up but currently Halftracks are swallowing all spare cash .