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JamesK if you aren't tracking your car and aren't too fussed about ultimate ride quality then just get the 19" wheels you like the look of. They will add a lot of weight unsprung and rotational weight but if you like how it will look then go for it. I have seen a staggered 19" setup that looked awesome on an 86 but have a read of the below thread and understand the negative consequences of big wheels.
http://www.ft86club.com/forums/showthread.php?t=67345
As for spring rates, I have read that MCA tend to have softer rear spring rates in general. If you aren't tracking the car will it really matter to you? Also keep in mind that replacement coilover springs are relatively cheap. If after three months you decide the car has more understeer (with a good alignment) than you like then spend $200 on a pair of stiffer springs.
Get some opinions from the forums but take them with a grain of salt. Do your own research and make up your own mind based on your own goals and how much performance you will compromise for looks.