I'm sure that you know, but just in case... You can only use 2wd springs on a 2wd, 4wd springs won't work. Also 98+ springs won't fit on a 83-97.
What is the stiffest stock front coil spring that will fit my 83 ranger?
If there are any differences in spring rate the stiffest stock spring would be either from a 97 model with extended cab and V6 or from a Splash Ranger. The '97 is the newest that will fit, so least degradation in spring rate due to wear, and extended cab with v6 was heaviest package so it would have the highest spring rate. The Splash package was lowered 1" from the factory VIA drop springs, and usually aftermarket drop springs have a higher springs rate, so it stands to reason that those would as well.
If you don't have to have stock I would probably go for a set of stock replacement progressive rate springs. Ya gotta figure that at this point the newest stock spring that will fit is 17 years old and has probably been ridden hard and well worn.
Does anyone know the spring rates of all the stock coil springs ford made for the 83-97 ranger also?
From what I've understood in the past they may have used one spring for all Rangers. I've definitely never seen front spring rates listed for pre-98 trucks.