Adjustment of the ride height is the easiest way you can move the A/S% with the C4 IRS. It's a huge tuning tool for drag racing. You want to raise the rear ride height and lower the front....

Those triple adjustment Vikings would work fine but so would their regular double adjustment models.... anything is better than factory replacement stuff. Everything has a designed purpose and limits thereto.... I can tell you from the time racing my 84 on the IRS, once it got pretty fast and started leaving violently, the switch from the stock Bilstines to the Double adjustment Vikings made it a totally different car.... Recently when I switched from the Double adjustment Vikings to a purpose built custom valved Penske - again it completely transformed the way the car handles.

As far as coilover or not.... whatever floats your boat. It's easier and pretty cheap to adjust spring rates for specific purposes with a coilover. For a couple hundred dollars you can have different spring sets for drag strip and road racing.. and can swap them in a couple hrs at most. The later yr C4s are sprung so soft.... you really need stiffer springs either way. Yrs ago when I started with the 84 build, I specifically found a z51 to build for the rear spring. I never intended to go as fast as I ended up going with the car.... But I was able to upgrade a little at a time because I started with the best possible option. You're car needs everything so you got to weight out the cost of coilover vs finding a new leaf spring and shocks and how that choice is going to work with your other uses for the car.
Will